My previous two could easily clear the 4' without touching it, so the lack of lateral support doesn't matter. They both could jump over 6' fences made of wood or chain link, but the style you posted in the OP @ 6' would probably keep them secure, unless you have a digger. The thing is, once they learn how to jump a 4' fence, they'll more than likely try the 6' as well. Best to go with 6' from the start.
Last edited by Nigel; 02-26-2014 at 06:32 PM.