You absolutely can, though we do not recommend using external drives with the Pi unless you have a 5W power supply (not the default Pi power supply), the SSD is self powered or uses a powered USB hub, and the cord connecting the SSD to the Pi is very high quality.

This is becuase the Raspberry Pi 4 is notorious for not providing enoguh power to the external SSD, which can lead to unreliability or even data corruption. That is why Start9, Umbrel, MyNode, etc have all moved away from the Pi to other hardware.

HOWEVER, the Pi is perfectly fine if you do not use an external SSD. Instead, we recommend using a 128GB or 256GB high endurance microSD card, no SSD at all. Start9's Server Lite is exactly that, a Raspberry Pi 4 (8GB) running entitely off the microSD card. Pretty cool.

Obviously, you cannot fit the entire Bitcoin blockchain on 256GB, which means your full node will be running pruned, which is perfectly fine. It will prohibit you from running Electrs and Mempool, that's it. Everything else on the Martketplace will work great, including LND, Core Lightning, BTCPay server, etc, etc