Four days a week is a very common training style. If you do want to experiment, and try twice a week for a muscle group(s), than help yourself, your call.
Most guy's will box themselves in with the idea of having to do so many workout in a weeks times. Why that seems to be written in stone I do not know. Actually a training plan of 9 days, as opposed to 7 days (a week), can work so much better for a lot of guy's. Allowing more important time for recovery, growth and strength. Might consider that option. Good Luck.