Do yourself a favor and install the driveshaft yoke into the tail of the trans, push it in till it bottoms out.
Pull it back out 3/4 to 1" from it's bottomed out position in the tail of the trans and measure from the center of the ujoint cup on the front of the drivshaft to the center of the yoke that the rear ujoint sets in and that would be your length.
You can just cut 4" off but you might want to check the actual length.
Once it to short its gonna stay that way.
You will want to do this with the cars rear end on jack stands not with the rear end hanging..