What justifies the V4 Plus being worth $350? They're using the CM4 so they’ve made a PCB, but what hardware are they adding over the peripherals available on a Pi 4/5? All I can tell is an additional Ethernet port, a SIM card tray, and an “ATX controller”.
What does the board look like, why can’t I DIY that version, etc. Are they just trying to make it up with the software (that I also can’t tell what it looks like).
It's not really worth that much. You absolutely could DIY it, probably just kludge in a basic $30 HDMI capture card. Also JetKVM is now just as "open-source" as PiKVM is, so there's not even a moral high ground to spending extra. Both are open-source software but not open-source firmware or hardware (no schematics or gerbers or anything like that available).