Arnold was known for burying his training partners. He's one of the rare exceptions that possessed an extremely resilient nervous system and a fiber type that allowed him to be really strong, yet have amazing set-to-set endurance. He also had a pain threshold second to none.
Arnold was one of the rare people who could hit failure on 2 or 3 sets of an exercise and then proceed to make his fifth set the best one. He could also tolerate workouts lasting hours, hitting as many as 60 total work sets.
I don't recommend Arnold's high volume approach, but the high-frequency approach is golden, especially for the natural guy or gal. Arnold routinely hit every muscle group three times a week, which may well be the optimal frequency for maximum growth and muscle retention while dieting down.
Understand, though, that for a natural bodybuilder, volume and frequency are inversely related. You shouldn't do both high volume and high frequency. Even enhanced lifters choosing a high frequency would do best to lower volume.
Do either a whole body approach 4 days a week or an upper/lower split done 6 days a week. With that amount of frequency, you only need one exercise per muscle group and three total sets: one set of moderate intensity to get ready, one demanding but conservative effort, and then one hard set.
Use different methods or sets/reps scheme (and exercises) on the 3-4 different sessions.