There is a little bit of softness hiding in these shots - all barring number 2 which is really sharp. I think you might get away using the unsharpen mask to try and correct it in the others (even if you can't "correct" the main version websized you should be able to get it to work)
Composition wise these are 4 fantastic shots! Each one is well composed and is drawing the viewers eye to the right parts with few distractions (though I would be tempted to crop a little off the left of shot 3, just a little bit too much of nothing).
I am guessing that these were hand held shots - with such good composition I would really consider tripod shooting (yes using a tripod in a zoo) if you can - that would certainly help get that extra sharpness
I must say number 1 reminds me of the (overdone a little now) artistic pose of a caveman holding his spear - you have managed to capture almost the exact same post with a gorrila - its not a branch its a spear he is holding now

Great shots and hope to see more