Some horses just have such a naturally terrible canter - if you want a competition prospect, a horse with a poor canter is generally one to avoid.
If the horse is a naturally good, correct mover the second thing to look at for 4 beating in canter is soreness. If a shoulder, back or hind end is out, the canter quality will suffer and often become 4 beat.
After pain, its the rider's fault for not riding/working the horse forward. In Dressage we see 4 beat canters a lot when riders try to bring the horse into collection by pulling on the reins and slowing the canter, rather than quickening the hind legs and shortening the stride by riding the hind legs up and quick.
To resolve this, the horse just needs to be ridden super forward for a while. We're talking get up off it's back and just let it roll along in canter while the rider has some lessons on how to ride forward!
Perfect! My guy will four beat under saddle - sometimes, and it feels awful! He has a nice rocking horse lope, but since the weather changed and we are in the indoor arena I think he feels a little cramped! When I feel him four beat I keep after him to return to a three beat. It's not all the time, but I don't like it! I've only just put my spurs back on.....and it hasn't happened for the last couple of rides.
He also does it intermittently on the longe, and I get after him for it and make him stride out.
I've been working on collection with him, picking up the reins and applying leg at a trot to push him up into the bit and as soon as he gives me a few strides of 'soft and round' I release, same with the lope....but I think the impulsion is lacking at the lope while trying to collect him, guess my legs need to work a bit harder!
I don't think he is very strong over his top line yet, even though he appears to be that way (very solid strong looking horse - a little fat)