Four month old regression?

So previously my baby sleeps quite well, waking up every two to three hours to feed. After feeding he will almost immediately go back to sleep. However as he approaches his four month old mark, when we feed him at night his eyes will be wide open, sometimes smiling at us as if expecting us to play with him. Is this regression? I am the kind that latches him back to sleep as I don't wish to pace up and down and sing song because he is almost 8kg heavy. Also I have another toddler sleeping in the same room with space so there's this problem of space constraint too. Anybody experiencing the same thing and how long does this last? Also, I do all the usual, lights out, no talking no playing but still baby is very awake. Latching to sleep sounds like a very bad thing to do and also he wakes up almost every 30min at night even after he falls asleep.