Popped a boil on my 4 year old's butt. It had come to a head and was draining on its own a little. I followed with antibiotic ointment. Was this bad?

Small abscess. Your 4 years old has an abscess on his buttock ,application of bacitracin ointment is o.k. and boil may subside .but if it becomes big or continues to drain ,then consult your doctor for more treatment like antibiotics and incision .
Staph buttock boil. Most likely this was a Staph infection which likes to "attack" the buttock area. Drainage is a good treatment - which is what a doc might do if severe.. Getting to a "head" helps drainage. I agree that a little antibiotic ointment to clean things up is a good idea - bacitracin or bactroban (mupirocin). Important to limit spread among family/your kid by washing clothes / towels in contact with infected area.