How do you cut a double lead thread on engine lathe.I have to cut a 4 pitch acme thread with double lead.

Your lead will be twice your pitch, i.e. 8

You cut one thread then move your tool along Z by the pitch and cut the other.

If you are familiar with the chasing dial depending on the lead on the leadscrew on the machine you can cut both thread by taking the cuts at opposite points on the chasing dial; this splits the lead.

Another method is to cut the 1st lead, then remove the drive gear and turn it 180 degrees and reinstall the gear.
if you are cutting between centers and using a driver dog with a face plate you can also index the driver dog to the opposite slot on the faceplate.
Geof's 1st suggestion is preferred.

Thanks!