The "trope", "tra-canter", or the "cripple-canter" looks something like this:
Or this (watch closely on the third horse especially):
Often, the four-beat lope or canter is the result of forcing "collection", with the incorrect use of the hand, gadgets (like draw-reins, et cetera), or even lameness.
I rode a dressage warmblood that was over-ridden in draw reins, and she had a four-beat canter.
By the way, you have no idea how weird it is watching Western Pleasure while listening to Metal. Too weird.