4 legs = Bit of a nightmare!
But... with a little patience, plenty of reference, and a lot of practice then it gets a little easier. No, scratch that. Your results get better, but it never gets any easier! hehe.
I'm starting to become a bit of a 4 legged guru, as I seem to get all the 4 legged stuff at work. (Buffalo, pigs, elephants... the hardest is the cat!) I think one of the main problems when it comes to animating 4 legs is making sure that the rig deforms the mesh in the right way. I've seen some rigs (been expected to animate with rigs) that are so anatomically different from the real thing that it's impossible to work with. In fact it was four legged animals that first got me into character rigging, when I was a handed an animal that was rigged like two guys in a pantomime horse suit!
Anyways, that was an aside... but pretty important to get the thing to behave correctly before you start animating. Difficult to tell with this wrinkly little guy (nice model and texture btw) but he looks like he's bending in the right places, (those back legs do bend back too far at the knee though) A little more movement is needed in the ankle / wrist areas (the front legs bend back a little at the 'wrists' as the weight goes down - that's quite a difficult one to rig right) the back legs look very stiff as the weight goes down too so you should watch that.
As for reference, you can't go wrong with Muybridge - his work is so useful for any animator! And to answer the queries about the what foot goes up and when, totally depends on what the animal is doing at the time / how fast it's going / if it's turning etc. Though there are a few distinct gaits when it comes to 4 legs. walk / trot / run (or canter) and gallop.
And this site pretty much explains them all.
http://www.oricomtech.com/projects/legs.htm
I think for your anim there's a lot of work needed on getting the legs spaced out right and the movement of the upper body synced with the legs. To me, if I just look at the upper body. it looks like he's happy and trotting along, yet his body is moving forward slowly and his legs are gaited as if he's in a walk.
Anyways... I've gotta go back to animating myself.