4 or 6 inch? Hmm, much can be said for either one. Neither is perfect, but both are useful. As an offense/defense weapon and one to tuck in the belt, I'd opt for a 4" model, as the shorter barrel is able to be drawn quicker and is more convenient to carry, and the shorter barrel gives an adversary less to grab on to when up close and personal. You will get better performance out of the 6" barrel for a given load, however. And the longer barrel gives a longer sight radius and a more weight forward balance, which is advantageous for target practice as well as hunting. That said, my 4" one is plenty accurate, and is quite capable of handling just about anything I'm liable to run across here in the southeast (except for black bear, for which it is at most marginal).
In the end, you have to weigh these factors for yourself. Is the revolver going to be primarily a SD gun? If so, you would be better served with the shorter barrel. Are you going to just be using it as a bedside gun from a SD standpoint, but do a lot of target shooting (esp. long range) and hunting with it? If so, you would be better served with the longer barrel. Personally, I found myself facing the same quandary and I opted for the shorter barrel.
About the 7 or 8 shot cylinders -- well, mine is a 6, and I don't feel undergunned. I respect the rule of 3's, in that SD encounters are over in 3 seconds, are usually 3 feet or less in range, and the matter is settled with 3 or less shots. You aren't going to get anywhere near 6 shots at once in a hunting situation, so those extra rounds don't help you there, and unless you are doing the tactical games sort of target shooting, you won't need to shoot any more than 6 shots at a loading there, either. Just my opinion, but I don't particuarly find myself wanting any more shots in my revolver than the 6 I've got. YMMV.