The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine . 
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.