The issue that many Forza 4 players had been seeing where online lobbies would crash has been addressed by Turn 10. A new patch just released from Turn 10 (that you will get the next time you play Forza 4) resolves this issue.
According to Turn 10 Forza 4 headman, Dan Greenwalt, by way of the official Forza 4 web site, the issue was caused when liveried cars were used and players jumped between lobbies. This triggered a slow memory leak condition that resulted in longer load time, which the crashed box would time out. The leak would spread to other boxes in the lobby, even if the other players were not using a custom livery.
Due to the complex nature of the issue, it was difficult for Turn 10 to reproduce the issue and run down the exact cause of the problem. Then, once the problem was isolated, they needed to test the new code to make sure that it was not causing any problem before pushing it out.
Besides fixing this major issue, the update also increases the level cap from 150 to 999. The performance index of 38 cars was tweaked and a bug that many complained about that was slowing navigation was also fixed.
