A yard is 3 feet....a horses canter stride is 4 yards or 12 feet. A yard is roughly a good human stride...so 4 strides between poles will give you a good canter into a fence
If you are jumping BSJA (or aiming to !) you should be looking for a 12' stride in canter. Course builders will always build to that standard in horses. They can't vary the course according to the size of your horse
If you are going to use canter poles it's best to use planks rather than round poles. This is incase the horse stands on one as he's going through them there is less chance for him to injure himself......just a tip from Tim Stockdales videos.