Open TRAIN.DAT and EXTENSIONS.CFG with your favorite text editor.
The TRAIN.DAT #CAR section for an 8 car train should look like this:
#CAR
22.5
8
0
0
16.29
1
Change the 8 above to 4, save & close.
In EXTENSIONS.CFG, delete entries for Car4 through Car7, save & close.
That should do it.