After about a year of weight training, you'll be ready for Advanced Bodybuilding. Typically, you will have made a HUGE improvement in your body over the course of your first year of training, after following the Beginning and Intermediate bodybuilding phases.
Now, you're a veteran weight trainer. You most likely have experienced some plateaus in your progress; it's in this phase where you learn a WHOLE lot about getting out of training ruts, sticking points, and leveled-off progress.
You have learned how to listen to your body's own bio-feedback and you know how your body best adapts to weight training, diet, and rest. By now, you should know EXACTLY what your body needs in terms of caloric intake, rest and sleep, and supplementation.
Advanced bodybuilders can continue on the 4-day split routine you started with in the Intermediate phase, or you can go to a 6-day split, where you split your body into thirds. It's up to you and your body.
You should also add in more isolation movements and do more sets. Typically, you can do 2-3 exercises per bodypart for 3-5 sets per exercise. Stick to 6-10 reps for building mass, 10-15 reps for building endurance and cutting up (reps should be higher for the legs — 10-15 for mass, 15-20 for definition).
At this point, too, your body has become accustomed to just about anything you've thrown at it, so you'll need to change things up quite a bit to “keep it guessing.” For instance, when training the pecs, one day you can train for 5 sets of 10, the next chest workout you can train for 2 sets of 6, 1 set of 8, 1 set of 10, and one set of 15.
Variety is indeed the spice of the advanced bodybuilding life. It will allow you to keep growing and your muscle “quality” will get better and better the more you change your workouts.
If you ever get to the point where you can't make gains NO MATTER WHAT, try the Hard Gainers Workout, switching to a 3 times per week regimen that's heavy on calories, supplementation, and High Intensity Training (HIT).
Image credit: License for re-use – https://i.ytimg.com/vi/zEK3_igC95k/hqdefault.jpg