It ain't gonna be cheap no matter which way you do it, but here are $ome option$:
-An accelerator like the Cyberstorm MKIII or PPC board will give you up to 128MB.
-DKB 3128 for up to 128MB. Rare as virgins in Las Vegas.
-Phase5 Z3 FASTLANE SCSI controller. Possible for up to 256M extra RAM. Key word:possible. Not likely, but 'possible'.
-PP&S Pro RAM Zorro board. Can add up to 64M via 30 pin SIMMs. The jumpers must be set a certain way to work in the A4000.
None of these will be cheap, unfortunately. You're best bet, honestly, is probably to find a rev2.4 Z3 Fastlane froma reputable seller. Good luck.