I'm kind of torn on TS4 horses. I love horses, and I am all for more animals in general. I'd love small animals that actually had interactions beyond feed/watch.
The idea of horses in TS3 was great, but the execution was horrific.Their pathing was a nightmare, even on home lots. Watching them make an 18-point turn to angle themselves to interact was painful to watch. And I'm not entirely sure they could do much better in TS4. They are huge objects.
I'd like to see the cows and chicken objects from ts3 come back, and it was a nice way to handle another large animal (the cows). But a static horse would cause an uproar to rival toddlers, I'm sure.