Two things to consider....
You have two distinct things to take into consideration, the first is if you want a 6 shot or a 10 shot, if that drives you and you want a 10 shot, then the -4 is the answer.
Past that - if you don't care how many cartridges it holds - the other consideration is "enhancements", or engineering changes, generally (not everyone will agree), the later engineering change will have some "fixes" that the factory implemented for whatever reason to improve the gun. In the case of visual differences (again, aside from the cylinder capacity/number of flutes), the -4 engineering change has the later rear sight assembly that, to me anyway, looks more "finished", given that it is inset into the topstrap neatly. This rear sight assembly also has a larger screw that attaches it to the frame, which is a plus, the older style - smaller - screw can be problematic, they can loosen up and without a proper screw driver can be a challenge to get tightened properly.
There are probably other, non-visual, differences, but the rear sight is the most notable differenciation, and a big plus. Unfortunately I'm not a 10 shot guy, so I'd look for a -4 that is a six shooter....
Good luck on your decision.