Its possible to get four instructor/black belt ranks. Perhaps not likely, but possible.
Here at my school I had one young man get the following ranks in the last ten years. He may not meet your criteria for four instructor/black belt ranks, but I suspect they come close:
2nd dan black belt in Tae Kwon Do (under me)
Black belt in Modern Arnis (Remy Presas)
Apprentice Instructor in JKD/Kali (Dan Inosanto)
He was given the title of "sifu" by our T'ai Chi instructor, who is himself certified by William Chen.
In addition to this he came within a hair's breadth of getting his Jagabaya status in Pencak Silat Mande Muda under Ibu Rita Suwanda. Her problems with her passport status kept that from happening.
He's come within a hair's breadth of getting his dan certification in Hapkido, but lost interest, I guess. He's now a 1st gup.
This young man's roomate earned the following:
2nd dan black belt TKD (under me)
Apprentice Instructor (Dan Inosanto)
Official Rep of Erik Paulson in Combat Submission Wrestling
His college courses and relationship somewhat slowed his training as of late, but he's trained in BJJ and Judo as well as the other arts listed. He didn't get four, but he might well have had his interests led him to branch out even more broadly the way the first did.
Now, both of these young men had some things going for them that enhanced their training and advancement:
1. Youth. Both started training in martial arts as pre-adolescents (7 and 11 respectively). They're now in their early twenties. One has time in middle school, high school, and college to do such things...if one makes the time.
2. Time. Their relationships with women didn't impinge on their training, nor did their social lives at school or in college.
3. Drive and ambition. Both not only trained here after hours, they went on an active seminar circuit and traveled as far as Texas and New York for training with instructors like Leo Gaje, Burton Richardson, Dan Inosanto, Remy Presas, Erik Paulson...the list goes on. They've spent thousands of dollars doing this.
4. A school that offers more than one art.
5. Supportive parents.
6. Supportive instructors...in other words they didn't encounter any stylistic bigots who slammed them for cross training or dissed the other instructors they sought out. I don't tolerate that here at my school.
It can be done...it may not be done often.
Regards,
Steve