Yep, I beat Forza Horizons, after getting it for Xmas. Not that it was hard, but it's difficult finding time to play when you're a parent and the sole-source of income in the family. I ended up playing only on the Saturdays, in the morning. I also happened to have picked up Forza 4....back in January.
I've played some of 4, and it is a bit more difficult than Horizons. But Horizons is a great game, I thoroughly enjoyed it. I got a great kick while driving my Trueno, upgrading that to compete against the other cars (like Ferrari's and Lambos). Eventually the speed was too great and I couldn't really control the car easily, so I had to change to a AWD car, :). A Skyline R34, which I then used to literally dominate any race I entered. Of course, this was before I figured out that the right trigger wasn't a binary switch, but was sensitive to how hard you pressed it, making acceleration a bit easier.
Despite not being a straight up simulation, it is a fun game that anyone curious should check out. The selection of cars is limited, but it is diverse enough. Plus each race has different requirements like a specific car brand, car rating, or drive type; to keep the races fresh. It's worth the money, IMO.
Now the big choice, do I try to get the achievements I missed or do I just go ahead and play Forza 4? :)