This bodybuilders diet is completely safe. It is based on giving your body the proper nutrients to gain muscle without taking a million supplements. The muscle building diet will have you eating natural foods and shopping along the outside of the grocery store avoiding heavily processed foods.
Arnold worked out six days a week back in the day because he could still make or maintain gains. Obviously when this was said “Train no more than 4 days per week.” it was targeted at “individuals who have a hard time gaining weight”. You don’t workout as much if you can’t gain weight. You need to burn less calories or eat more to gain weight while working out. Obviously you cannot workout 6 days a week and gain weight if you’re burning too many calories or not eating enough. Therefor, workout less days to gain weight because you’re not as awesome as Arnold.