Is it safe for my four year old to take macrodantin (nitrofurantoin) 50 mg daily?

Probably yes. For a urinary tract infection, the total daily dose of 50 mg would be appropriate for a child weighing about 20 pounds. For prophylaxis (prevention) of infections, 50 mg would be an appropriate for a child weighing 50-100 pounds. That would be a really really large four-year-old.