I have been a huge fan of the Forza series but 4 is basically a slightly upgraded Forza 3. Same old tracks, a few different vehicles, enhanced graphics that remain probably the best in the racing genre but definitely feels like the same old, which is not necessarily a bad thing.
This series is still the simulation racing champ in my opinion but got rather stale. I bought GT5 and tried playing it and it comes nowhere close to the Forza series.
The mediocre score comes from the DLC. I do not joke when I say that at least over half the cars require a DLC {Day 1 DLC nonetheless} pack which is asinine for a simulation racer. Want any Porsche? You will have to pay for it which I found laughable. Vehicle choice is a simulation's game biggest strength and Forza 4 absolutely cramps the choices to only a handful unless you want to pay on top of the original price tag for cars you should already have. This was immensely frustrating. I would highly recommend Forza 3 over 4.