Did you know that the United States Postal Service (USPS) supports several types of centralized mail delivery systems for incoming and outgoing mail? Two of the most common USPS-approved mailbox types are 4B and 4C mailboxes.

4C or 4B Mailboxes? Knowing the Difference and Why It's Essential

Did you know that the United States Postal Service (USPS) supports several types of centralized mail delivery systems for incoming and outgoing mail? Two of the most common USPS-approved mailbox types are 4B and 4C mailboxes.

Many business or multi-unit property owners wonder which of the two types is better for a centralized or commercial mail delivery system. Is there a difference between the two types?

In this post, our professionals from Mailbox Depot USA will explain all about 4B and 4C horizontal mailboxes, including their major differences and the benefits of switching to a centralized mail delivery system.

What Are Horizontal Mailboxes?

Horizontal mailboxes are ideal for commercial and residential applications where a large number of people work or reside, like an office building or condo complex. The USPS commonly utilizes this mailbox type for centralized mail delivery systems, which consist of a large structure with multiple units for each separate mailbox and parcel lockers for large packages.

Horizontal mailboxes are suitable for indoor and outdoor use and come in varying shapes, sizes, materials, and configurations, such as 4B and 4C mailboxes. The most common applications for horizontal mailboxes and centralized mail delivery systems include:

  • Apartment and condo complexes
  • Residential and multi-family homes
  • Industrial and commercial buildings
  • Offices and retail buildings
  • High-security institutions
  • Government or military buildings
  • Schools, colleges, universities, and dorms
  • Hospitals and healthcare facilities
  • Shopping malls and retail centers

Let's look at the distinctions between the 4B and 4C horizontal mailboxes.

4C vs. 4B Mailboxes: What Are the Differences?

So, even though these two types of mailboxes are quite similar, what is the difference between 4C and 4B mailboxes?

4B Mailboxes

4B mailboxes, also known as Standard-4B or STD-4B, are USPS-compliant and consist of a single wall-mounted unit with multiple mailbox compartments and parcel lockers. STD-4B units include both front-loading and rear-loading mailboxes. The USPS first implemented the STD-4B mailbox in the 1970s, and they are one of the most common types of mailboxes for centralized mail delivery systems.

4C Mailboxes

About 2004, the USPS switched to a new standard, the STD-4C. Now, any multi-unit building with permits from after October 2006 must use 4C mailboxes. However, any buildings with 4B mailboxes that were in place before the 2006 transition can continue to use the STD-4B design when replacements are necessary.

USPS Standard 4C (or STD 4C) mailboxes are wall-mounted and centralized. Since 2006, the USPS has required the 4C for buildings undergoing major renovations or new construction.

Essentially, these mailbox units feature an improved design over the previous 4B mailboxes, with compartments that are flatter and more horizontal. In addition, these wall-mounted mailboxes can come in front-loading or rear-loading units, depending on the application and the purchaser's preference.

The new USPS STD-4C mailboxes are different from the STD-4B design in a few ways. To start, they're stronger and heavier. They have better security, such as an upgraded residential lock. The mail compartments are also bigger to better accommodate larger packages. However, each unit also has an accompanying parcel locker for oversized deliveries.

The current USPS STD-4C standard requires one parcel locker for every five mailbox units. (Before 2021, the parcel locker requirement was one unit for every ten mailboxes.) The mailboxes and parcel lockers will ideally have a central location close to the entrance door of the building or complex.

STD-4C Horizontal Mailbox Mounting Options

USPS-approved 4C mailboxes have a variety of mounting options from which to choose. When implementing a centralized mail delivery system for multiple residences, determining the right mounting style is imperative. Below is a quick summary of the different options:

Recessed STD-4C Horizontal Mailboxes

Recess-mounted 4C mailboxes are the most common type and consist of a unit that is set back into a wall or structure. Front-loading units have a master door, and rear-loading units require installation with a secure mailroom directly behind them.

Surface-Mounted STD-4C mailboxes

For applications where a recessed 4C mailbox isn't suitable, a common alternative is to install the unit using a surface-mount collar. A surface-mount collar is a device for front-loading surface-mounted mailboxes, allowing the mailbox compartment to recess into the collar. This type of mailbox type is suitable for any building with wood-studded walls.

STD-4B and STD-4C Mailbox Specifications and USPS Regulations: A Quick Comparison

Although residential and commercial 4B and 4C horizontal mailboxes are similar — essentially, the STD-4C is the upgraded version of the STD-4B — there are some key differences. Here's a quick rundown of the different specifications for each standard:

USPS Approved Usage:

  • 4B: Private use, new construction, and replacing old units of the same size
  • 4C: New construction after October 2006, replacing old 4B units for USPS delivery, and replacing 4B units of a different size


  • 4B: Indoor use only
  • 4C: Indoor or outdoor use

Mail Delivery:

  • 4B & 4C: Front-loading or rear-loading

Mounting Types:

  • 4B: Recessed mounting
  • 4C: Recessed mounting, free-standing, surface mounted, pedestal kiosk (private use)

Security Requirements:

  • 4B: Tenant Cam locks with two keys
  • 4C: Tenant Cam locks with three keys

Master Locks:

  • 4B: USPS-installed master Arrow lock, private master lock, or combination lock (private use)
  • 4C: USPS-installed master Arrow lock (front-loading); rear-loading requires a secure mailroom but no lock

Outgoing Mail Compartments:

  • 4B: Integrated or optional access lock
  • 4C: Full integration or protective lock and anti-fish comb

Additional Accessories:

  • 4B: Collection boxes
  • 4C: Directories, collection boxes, key keepers and letterboxes for USPS delivery, recycling and trash bin are mounted on wall

Cluster Specifications:

  • 4B: Groups of 10 mailbox units
  • 4C: Maximum height of 56 1/2", 3 - 5 compartments high

Both mailbox types have further customizable options, depending on request.

What Are the Benefits of Centralized Mail Delivery Systems?

Centralized mail delivery systems using STD-4B or 4C mailboxes aren't just highly efficient but also cost-effective. In fact, the benefits extend not just to the USPS but also to residential and commercial populations:

Reduced Costs: Centralized mail delivery reduces the number of stops that postal workers must make, reducing costs and delivery time. With an annual mail volume of almost 130 billion, combining stops can result in a drastic cost reduction for the USPS. Also, installation costs for horizontal mailboxes are lower per resident than for individual curbside mailboxes.

Increased Security: With horizontal mailboxes and parcel lockers, both postal workers and residents can enjoy better security and safety thanks to a more visible indoor location, metal compartments, a door lock to protect incoming and outgoing mail, and a separate parcel locker for packages.

Lower Environmental Impact: By reducing the number of mail delivery stops at residential and commercial mailboxes, postal workers don't have to drive as far, reducing greenhouse gas emissions and carbon footprint.

Greater Convenience: Both regular residents and commercial workers enjoy greater convenience when their mail is delivered, as they only have a short walk outside their door to their mailbox unit and parcel locker. In deciding between indoor and outdoor, an indoor location protects against the weather.

USPS-Approved STD-4B and 4C Mailboxes from Mailbox Depot USA

Whether you need to install STD-4B or STD-4C mailboxes for a residential or commercial application, we've can take care of your needs here at Mailbox Depot USA. Our horizontal mailboxes are strong, durable, cost-effective, and fully compliant with USPS requirements.

Browse online to search through our catalog, or reach out to our customer service with any questions.

By Admin Staff WLS