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Salsbury Mailbox Blog

What Are the Differences Between 4C and 4B+ Mailboxes?

December 20, 2022

When you are looking to install a horizontal mailbox in your facility, there is a lot to think about. One of the biggest questions you will have to tackle is: What are the differences between 4C horizontal mailboxes and 4B horizontal mailboxes? It’s easy to get lost in Google searches without getting any closer to knowing which type of mailbox will suit your facility best.

Fortunately, Salsbury Industries is here to offer you a comprehensive guide on what each type of mailbox is and how to choose the best one for your business, apartment complex, or whatever other type of development you own or manage.

Here are the differences between 4C and 4B mailboxes and how to know which one is right for you.

What is a 4C Horizontal Mailbox?

USPS STD-4C mailboxes are centralized, wall-mounted mailboxes that are required for all new construction and major renovations. A “major renovation” refers to a situation in which the wall where the existing mailboxes are installed will be significantly modified.

Salsbury Industries carries a line of USPS-approved 4C horizontal mailboxes that would make a durable and attractive addition to your facility. These mailboxes are well-suited to multifamily housing units, apartment complexes, commercial buildings, government and military bases, schools, colleges, and other residential and commercial developments.

Our 4C horizontal mailboxes are constructed of heavy-duty aluminum and stainless steel hardware and come in configurations that are 1 door wide (single column) and 2 doors wide (double column). They are available in bronze, sandstone, aluminum, and black. Your building tenants will feel very secure with our mailboxes’ built-in heavy-duty cam locks and anti-fish plates to prevent tampering. The USPS requires every 4C mailbox configuration to have 1 parcel locker for every 5 mailbox doors, which will give your tenants an easy way of getting larger packages delivered. All Salsbury 4C horizontal mailboxes come with a 5-year warranty.


Our 4C recessed mounted mailboxes come in four different heights: 6 door high, 10 door high , 11 door high, and Maximum Height units. They are available with front and rear loading compartments and are able to be used for indoor and/or outdoor centralized mail delivery. Each mailbox features a compartment for outgoing mail collection that will prove very convenient for the people who live or work in your building. The outgoing mail compartment features a weather protection flap and mail slot.

Mail carriers access Salsbury surface mounted 4C mailboxes from the front through one or two easy opening master door panels. 4C surface mounted horizontal mailboxes include a USPS-approved 4C mailbox with a matching surface mounted enclosure. Like the recessed mounted mailboxes, these have a compartment for outgoing mail collection with a weather protection flap and a mail slot and are suitable for indoor and/or outdoor centralized mail delivery.

The 4C free-standing mailbox includes a USPS-approved 4C mailbox with a matching free-standing enclosure. This mailbox is easy to assemble. In addition to many of our other 4C horizontal mailboxes’ features including a compartment for outgoing mail collection with a weather protection flap and a mail slot, free-standing 4C mailboxes will allow you to install 4C mailboxes in places where a wall mounted application isn’t desired or available.

What is a 4B Horizontal Mailbox?

Now, let’s get down to the differences between 4C horizontal mailboxes and 4B horizontal mailboxes. While 4C horizontal mailboxes are required for new construction and major renovations, 4B horizontal mailboxes are for direct one-to-one replacements. The USPS permits the use of 4B horizontal mailboxes for replacements as long as no changes are made to the rough opening where the existing wall-mounted mailboxes are currently installed. You should always check in with your local post office to be sure, though, before you purchase any mailboxes.


Salsbury 4B horizontal style commercial mailboxes are perfect for office buildings, apartments, condominiums, and residential developments. They are made of aluminum and stainless steel hardware. The mailboxes feature a powder coated finish available in aluminum, bronze, or sandstone. 3600 series 4B horizontal mailboxes are recessed mounted and come as front loading units or rear loading units. All 4B horizontal mailboxes are USPS-approved for replacement and/or retrofit installations.

Each door includes a five pin cylinder cam lock with two keys. The front loading door panel swings on a continuous hinge which will guarantee your mail carrier convenient delivery. Optional engraved placards, as well as custom engraving, are available for doors.

What's the Right Mailbox for Your Facility?

So now that you have the information, you may still be a little lost as to how to choose between 4C and 4B horizontal mailboxes for your facility. The biggest factor to consider is whether the mailboxes in your development would be a new construction, major renovation, or a one-to-one replacement. If your building is being newly built, that would be considered a new construction. And if you’re making significant changes to the rough opening where the preexisting mailboxes in your building currently are, that would be a major renovation. In both of these cases, you will need to purchase a 4C horizontal mailbox.

But if you are planning to get mailboxes that will take up the same amount of space as your existing mailboxes and aren’t going to make changes to the rough opening, you can purchase a 4B horizontal mailbox. As mentioned, all Salsbury 4B horizontal mailboxes are USPS-approved for replacement and/or retrofit installations.

If you own or are managing a building where you’re unsure if you want to make changes to the rough opening in the wall where the existing mailboxes are, consider the type of facility it is. 4B horizontal mailboxes are ideal for apartment complexes and other residential developments while 4C horizontal mailboxes are great for commercial buildings.

No matter which type of mailboxes you choose for your facility, know that Salsbury Industries will be able to suit your needs with our various lines of horizontal mailboxes. Click here to get started on your search for the perfect 4C horizontal mailbox, or click here to find the best 4B horizontal mailbox for your building.