While the server will work, there are some challenges.
Raspberry Pi 400 lacks the camera and screen connectors, specifically:
- 2-lane MIPI DSI display port
- 2-lane MIPI CSI camera port
so you can't use Pi screens and camera modules. You need to find compatible USB webcam if you want to make time lapse of your print or stream your build to oversee it. If you want to add a touch screen, you need to use HDMI+USB one, as most of the screens dedicated for Pi won't work without DSI port.
Pi 4b has:
- 2 × USB 3.0 ports
- 2 × USB 2.0 ports
Pi 400:
One of these will be, of course, occupied by the printer, so if you want to connect a touchscreen and webcam, with 4b you still have up to three usb ports free, when with 400 you are all out.
Built-in keyboard is somewhat redeeming quality, especially if you are old time console jockey and don't need a mouse or touchscreens, and 20% faster CPU is nothing to sneeze at if you want to use AI plugin to oversee your print. For end users not interested in any kind of hardware hacking, 400 being already assembled with case and cooling is a great benefit, too.
In summary: it's compatible, faster and more convenient for end users, but less hackable. It's also significantly cheaper during 2022/2023 Pi Shortage, sometimes being available at MSRP and in stock.