A bodybuilding workout will build muscle only if it stimulates growth and is followed by enough recovery time, sleep, and a sufficient supply of nutrients that permit the body to heal.
Train too often and you may not stimulate any growth because you’re unable to train hard enough. And even if you stimulate growth, you won’t have sufficient recuperation time between workouts to realize the full benefits.
If you’re young and have a leisurely lifestyle and good genetics for bodybuilding, you may initially make some modest progress from four or more workouts a week. But why not optimize your recovery ability, make more progress, and, as a bonus, spend less time in the gym?
It’s easy for a natural bodybuilder with normal genetics to overtrain. But someone with outstanding genetics for bodybuilding can prosper on more frequent training, and such a person can prosper on even more frequent training if he’s on bodybuilding drugs.
Typical lifters are best off training no more than three times a week; just twice a week is better for most. Never mind that many drug-assisted super-responsive bodybuilding champions prospered on six workouts a week. Some of them, for short periods, even prospered on twice-daily training, six days a week.
Such a high frequency is training suicide for typical bodybuilders. Most split routines are problematic for average lifters. The physiological system is so interwoven, and many exercises overlap in the muscles and other tissues they recruit.
Furthermore, hard training for just a limited area of the body still produces a systemic demand that you need to be recover from. If you train too often, you’ll never recover from systemic fatigue and will make little or no bodybuilding progress. Have more recovery days than training days!