Please note that while it serves the same function as the RPi4, this isn't an exact drop-in replacement.
Basically, should you need to burn an OS to the board, I'd like to point out that you don't use the Raspberry Pi OS. Instead, you need to use BTT's version (Google: OS System image for CB1). While it's open source and you can get a precompiled O/S image with Klipper installed, it's definitely not as well-supported.
So why a CB1? There was a time where shortages of Raspberry Pi made it hard to find one for a reasonable price. Also, it turns out that RPi's are overpowered when you want to dedicate a SBC for 3D Printing. CB1's are significantly less powerful, but they fulfill the job just fine.
Also, Mantas have a slot for these boards to plug in so you don't need to run a 5V supply and wires to a separate board. It's a bird's nest under my printer and without labels I need to trace which wire goes where. Having less boards and power supplies means having fewer wires. Who doesn't like that?