Amiga OS is low footprint, but if you're thinking of running large m68k applications emulated you'll need more memory than on a classic machine. Due to the way the emulation works.
Can you even BUY less than 128MB sticks nowadays? (I know SIMMS for CSPPC and BPPC are generally a bit smaller, but remember that on these systems you could dual boot with 3.9 to run old applications without emulation).
It's also expected that you'll be working with higher resolutions in all your programs, and with larger documents. I mean, you can easily play videos on the CPU power in an AmigaOne or a CSPPC, so why wouldn't you? This means you'll need some memory as well.
But a MINIMUM requirement to run OS4 without any JIT emulation should be somewhere around 8MB. On a 320x200 8 bit screen ;-) (most people don't actually know how much memory screens take up. Try to calculate memory for 32 bit graphics in your preferred screensize and you'll see that 32MB won't last for many applications at all... 1600x1200 takes up 7.5MB of memory)