Ok did some wiki search! MP4 stands for Marlboro Project 4! wahahaha!
Now its McLaren-Project 4.
The nomenclature for McLaren's F1 cars since the merger has caused some confusion among fans of the sport, as all McLaren cars since 1981 have carried designations of the form "MP4/x", or since 2001 "MP4-x", where x is the generation of the chassis (e.g. MP4/1, MP4-22). In fact, "MP4" stood initially for "Marlboro Project 4", so that the full title of the cars (McLaren MP4/x) reflected not only the historical name of the team, but also the names of the team's major sponsor and its new component part. The team's cars still use the same nomenclature, but since the change of title sponsor for the 1997 season, MP4 is now, rather conveniently, said to stand for McLaren-Project 4. At no time has the "MP4" prefix reflected the particular generation of the chassis.