I love horses IRL and would love them in TS4, but only if they're nothing like the TS3 horses. Other than the already mentioned problems, they were way too time consuming. Yes, I know that horses IRL are time consuming and that sims take longer time than an actual person would doing something, but at least for me in TS3 I could either play with horses and do hardly anything else or play with the rest of the game. I don't want my game to be just horses.
As far as the world goes, I think it'd be best if they were somewhat confined to a world. Not saying that I want them to make it impossible to have horses anywhere else, but we don't need packs of wild horses roaming the streets of suburbia.