Ammonia
I will take a shot at this here as i just fixed a tank (not mine) that had a 4 ammonia but a ph of 4.9 with living fish inside
when your ph was at 4ish range (yes it is possible) your BB stops or goes dormant and your ammonia stays non-toxic... by buffering your ph up you made your ammonia deadly and at the same time killed your BB that is why you got cloudy water...
buff the ph of new clean water in a clean bucket close to the current ph and do a 3-4 gallon water change, today and tomorrow then start to buff the ph back up slowly while doing daily water changes.