My son went through a phase where he did this (right around 4-4 1/2 months actually). I'd hear him on the baby monitor around 6-ish and think oh great, he's waking early, but then I would fall back asleep and I assume he did too because then I would wake up and hear him again around 7:30 (his normal waking time) I just kept with his normal A times and nap times and everything seemed to work out ok for him. He did have a rare morning every once in awhile where he would get SO CRANKY I would put him down early, but with him I've found it's the time I get him out of bed that matters, not the time he actually wakes up. As long as he was just laying in bed happy he was resting and that was enough for him.
Also DS's A times are sometimes longer when we're out running errands. I don't know if he catches a quick catnap while he's in his carseat or what (he's always awake when I go to get him out of the car), but I've seen him be perfectly happy on 3.5 hours of A time when we run an errand in the middle of that where if we're at home the whole time he's ready go to down at 2.75-3 hours every time.