I've done both, my first i formula fed and weaned at 4 months (ish), second had nothing but breast milk up until the day she turned 6 months, when we 'celebrated' with some mashed sweet potato 
My first...colic, reflux, bottle refusal, picky as hell (still barely eats), eczema, needs a preventative inhaler and allergies.
Second...no problems. None. Up to now of course, don't want to jinx things!! She's almost 2 and her and her brother are like chalk and cheese.
Their differences may have absolutely NOTHING to do with how I fed them and I must say, I don't see it making that much difference for a formula fed baby as formula isn't a 'natural' bodily fluid anyway.
If someone asked, I would say nothing but breast milk until 6 months, from personal experience, but there are millions of babies out there fed formula and weaned at 4 months who are healthy and fit as a fiddle.
Do what you feel is right, you know your baby better than anyone else. I was formula fed and weaned at 12 (!!!!!) weeks and I'm still here ;)