I have those. They are 4.75". I read on a board someplace that somebody had one open up while going over a jump, which she wasn't pleased about. Other than that, I haven't heard anything negative. Mine won't acidentally open.
Here they are on my Pro Jump. They are roomy. I think the 4.25" size would be adequate for my size 6 boots. I got them for use on a saddle that the leathers wouldn't easily slip off of. Edit: It was a dressage saddle that I no longer own, not the Pro Jump, lol!
I like the bent leg sometimes called Australian safety irons. That curved part might put a dent in your saddle flap. Either take them off in transit or handle the saddle carefully.
some comments: http://www.ultimatedressage.com/foru...afety+stirrups