I have each of these scopes mounted on separate rifles. While there is nothing wrong with the 6 x 36, I much prefer the 6 x 42. Mostly it's because my 6 x 42 has the HD (Heavy Duplex) reticle, which is quite good for low light hunting. I know that the 6 x 36 is a FX II and the 6 x 42 is the FX III. So, it stands to reason that the 6 x 42 will be somewhat optically superior, though by how much very much depends on the individual.
I hunt wooded areas, where the longest shot I'm likely to get is just over 100 yards. Also, the shooting lanes are narrow and you have to be ready to shoot quickly. I've not found the 6 x 42 to be a handicap, though YMMV. In fact in the past I've used some pretty good 4X scopes, like the old Nikon Monarch and IOR. Certainly a 4X scope in the kind of terrain I hunt is never a mistake. However, once I tried the Leupold 6 x 42, I've stayed with it and that was 3-4 years ago. If you are considering buying this scope, make sure you get a FX3 (not FX III), as this is the Leupold newest version with some improvements. I've personally not looked through the FX3 but from what I've read and heard, it's a real nice scope. Also, if you decide on a 4X scope, you might take a look at the Zeiss 4X Conquest. I've heard nothing but good about that scope, though once again I've not yet looked through one.
If I were you, I wouldn't agonize over whether to get a 4X or a 6X. If you buy a good brand name, either one should do fine. The best thing to do, if you can, is to look through these scopes first before you decide. That can make all the difference, despite what advice I or others give you. You may have a gun shop in your area that carries some of these. I've been quite successful at my local outdoor shooting range in getting to look through different scopes. Most of the people who frequent ranges (at least the range I go to) are friendly and don't mind letting me do this.
Good luck in your choice.