Yes, the A3640 is crap. Yank it out and sell it on eBay. (If it's a rev 3.1 or 3.2 it should sell for a good chunk of change. I sold my original 3.1 a few years back for $120!)
To get the T-Rex working you might need to set the INT/EXT jumpers (J100/J104) on the A4000 motherboard. AFAIK, they should be set to INT when using the T-Rex, whereas the A3640 needs them set at EXT.
