Are the two lines of pre-drilled holes in 4" french drain pvc pipe enough holes, or is it better to drill another line of holes, maybe between the 2 lines?
The holes in the pipe are enough; drilling more would weaken the pipe. The important thing with French drains is to have a good bed of gravel around the pipe for several inches. I like using a sock on my pipes to prevent the fine gravel from falling into the holes. No need to drill more holes the ones that are there are more than enough.