Fixed, are you sure GTAIV MP doesn't work?
I know the DeathRace map needs resetting. To do this max, type :
/resetmta 20 0 14 20
/resetmta nitro nothing IVGrid
As I was writting this post, I have realised what is wrong. GTAIV should be ok now, but races made will need to be fixed.
Report here, bug is fixed. Thanks max.