the works on World of Subways Vol. 4 have actually been completed since quite a while, but we still ran through the complete game structure to enhance the overall performance of the game. On December 22, 2015, we will release a performance update for WOS4 that will automatically be installed by Steam.
This update brings great improvements to the gaming performance, and no lags should appear when the game tries to load data during the drive.
As soon as the update is released, let us know how it works by writing an answer in this thread.
Edit: WOS4 is now available with 25% discount at Steam. Check it out!