Well, some accessible areas around lots like those on the island in Windenburg and in that neighborhood in Brindleton Bay where the 64x64 lot is located (forgot what it's called) are actually pretty big. When I had my Sims jog with their dogs there, they'd be gone for hours. Those locations would, imo, be big enough to ride a horse. If they dedicated an EP to farm animals and put horses in there, they could create a new world like Windenburg or Brindleton with large, nature-y neighborhoods for your Sims to ride their horses. They could place some obstacles in there and all... would be pretty fun, I think!
I absolutely love horses and unfortunately never got to play them as I didn't own TS3, so a part of me really wants them in TS4. But like others, I don't know if they'd be done right... so am not sure. But still, I'd probably be super, super excited if they did decide to do a farm pack with horses!