To be frank I have no personal experience of an A4000 and any of its graphic cards, 4mb just sounds too little to me.
It is too little. Way too little. Way way too little. Ppl complain about A1200 having only 2MB chipmem but A1200 only uses 8-bit gfx. Gfx cards use 32-bit gfx. So 8MB on a gfx card at 640x512 is equivalent to 2MB on AGA. IOW 8MB is way way way too small. 4MB is ugh.
But zorro III in itself must be quite fast.
For games it makes no difference. Games put all the gfx in the gfx card chipmem and then all gfx access takes place inside the gfx card. That is the whole point of a gfx card: to
not use the cpu to copy data around.