I have put just about every type of scope and optic I own on my CX4 (9mm) at one time or another. Generally speaking, you probably won't be shooting it much beyond 100 and very rarely beyond 200. The best groups I have gotten out of mine is 4-5 MOA at 100 yards with a 10x scope. With the "iron" sights it was closer to 8 MOA. If you don't aim at the same point each time (and my eyes don't allow for that with the funky iron sights), then your group size is going to be bigger.
Right now it is set up with a ATN MK390 NV scope and I can hit a 6x6" square at 80 yards consistently with Blazer 115 gr. ball ammo. I might be able to do as good at 100, but haven't shot it in the dark that far.
You can extrapolate from 25 yards with 2 MOA to 8 MOA at 100, but I have found that sometimes you can do better when you actually shoot the full 100 yards and have a way to aim precisely.
A 4 MOA dot optic is probably fine all things considered. Figure out the ballistics and I would be willing to bet that you would have a high (inside the 4 MOA dot) 25-30 yard zero that would keep the round inside the dot from 15-20 yards out to 80-90 yards or so. Of course, that will actually depend on the partcular ammo you are using and the actual sight radius height over the bore radius, but basically with a dot that big, it will be a good match for the capabilities of the gun out to that far. Place the dot on the target and that is where the bullet will go (somewhere within the dot).
In other words, I don't think I would spend extra bucks to get a finer reticle, but if you can find something with a finer reticle within your price range, that is great.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011