I think I've made an A4000T board boot without the disk module before.
Looking at the schematic, the only thing I can see which might stop it booting is an input from the DIP switches. You might need to connect one of these to make it like the switches are still there.
Switches 1-8 are on pins 1-8 of CN200. Pin 9 on CN200 is ground.
E.G to make switch 8 "on" then you'd link pins 8 and 9 on CN200 (CN600 on the motherboard). To make switch 4 on then link pins 4 and 9, etc.