You can go up to 16MB on the motherboard using 72-pin SIMMs. Either four 4MB, or two 8MB SIMMs will work. You can also buy a Zorro II or Zorro III card that you can add mem to. ZII is max 8MB but is 16-bit so it will be slow and it will slow the 4000 down, ZIII is 32-bit and will typically be about 80% of the motherboard RAM speed. Some types of 32-bit RAM are the DKB 3128 (128MB Max, 72-pin), Phase 5 Fastlane Z3 (64MB or 256MB with mod Max, 30-pin), and a ProRAM ZIII card? (64MB? can't remember ram types)
Your best bet is a CPU card with RAM slots, you can go up to 128MB of fast RAM which will speed things up considerably.
