That sounds extremely bizarre (to me anyway). I've never even seen a PH of 4.0 before, I would almost venture to guess your PH meter is defective. Do you have another one to compare your reading to by any chance? The fact that they look wonderful also makes me wonder even more about it because I wouldn't think they'd look that great if the PH is that far off but I suppose at Day 10 they're not really taking advantage of nutrients at this point too much (assuming thats day 10 from seed)
Good luck, hope you get it sorted out but if you have access to another meter I would possibly try that and see if the reading is any different, I would hate for you to keep adding PH up and have it be the meter.