Migrating ZynthianOS to 64 bits - #3 by jofemodo is the main reason why you might want an 8G machine, but the zynthian v4 is 32 bit so it doesn’t make too much use of the 8Gbyte machine.
there’s a bit more discussion on the subject here:
Personally I suspect ( but emphatically do not know) that we will use the 8G once we get the zynth onto a 64 bit OS…
So sorry there’s not a hard answer but hopefully it gives you an idea as to the issues.
Just for example I use a Pi4 8G as my desktop machine and it does use the memory for chrome pages but a 32bit OS machine can’t actually run one program with more than 3G and that is the sort of thing we would probably want to do with the zynth…