I agree with onecoyote as far as the killing power of #4 buck on coyotes within reasonable ranges. A coyotes chest is big enough to reliably put at least a few of those pellets though.
On foxes though, it's proven to be a dismal failure for me. Once the ranges get much past 25 yards you stand a real chance of just winging a fox in the leg, or worse, the guts. If you want to judge a shotgun load for foxes, set your maximum range at the point that you can "consistantly" put at least 2 or 3 pellets into a softball sized circle. That's about the size of the average foxes vitals for a chest shot. One #4 buck in that circle will kill a fox, but it'd have to be a good hit every time, and it's just hard to garantee.
I don't even use a 12 gauge anymore for predators. I started using a Remington 870 Express Magnum in 20 gauge. The 12 gauge is perfect for coyotes, but it gets right rough on a fox at any kind of closer ranges. It does make choke sellection a bit different though as the 20 gauge(mine anyways) seems to shoot tighter with any given choke. For instance, if I put in the Super Full Turkey choke, or even a standard full choke, and 3" magnum #4 or #2 bird shot, I can literaly riddle a 2' circle at 50-60 yards, but the pellets just don't seem to have enough power to be effective at those ranges.
My prefered load is the Federal 3" magnum #2 bird shot. I've counted them, and they average around 108-110 pellets. This load with a modified choke will cleanly kill a coyote up to around 35-40 yards, and will also drop foxes reliably at the same ranges. If I'm hunting somewhere that I doubt seriously I'll see a coyote, I switch over to the Winchester Supreme 3" magnum #4 bird shot, and an improved cylinder. This combo will bust a fox every time at 45+ yards, but not turn the hide to window screen at closer ranges. I've killed coyotes with this load too, but I try to put my bead on the head/neck, and that way the shot is concentrated from the chest forward.