The answer to your question is "it depends".
You haven't described why you want to use a stack, nor why you would use a 4 member stack vs. a 6 member stack.
As you note, you can stack up to 8 members, so either a 4 or 6 member should work.
Which to choose, would depend what the stack needs to support.
Some of the prior replies note there's no performance difference. Well, that's not actually true, although the performance deltas are such, most of the time, they wouldn't be significant.
What to keep in mind, a stack is physically a ring architecture, which can add latency as traffic transits multiple switch members, and can also encounter bandwidth bottlenecks (the latter, though, Cisco mitigates with evolving stack ring bandwidth increases). (Also, I believe current stack ring technology still floods multicast and broadcast.)
One advantage of a stack, each stack member is able to do its own forwardimg. However, believe control plane functions are only supervised by just one stack member. So, it's possible a larger number of stack members might overload the "master" switch.
Although my direct experience with physical Cisco stacks ended with the 3750 series, Generally, what seemed to work fairly well was to limit max member stacks for user edge devices. If used for servers edge, limit stack to half max. If used for distro or small core, limit to just a 2 member stack. This general rule is based on the fact that both individually and a ring topology are easier to overload, so using less stack members for likely busier devices, provided more headroom.
BTW, although a VSS has been renamed a virtual stack, it's a different animal, with different usage considerations, besides it being limited to 2 stack members.
Oh, although my direct experience with Cisco physical stacks stopped with the 3750 series, the company I was working for started to use Brand "J" stacks, which took a somewhat different architectural approach.
