Because babies sleeping patterns can change several times throughout the first couple of years, this is probably just one of those changes. You may just have to adjust your own hours of sleep, so you're not so tired from waking up earlier than usual. Try going to sleep around 10 or 11 and then wake up with your son in the morning and begin your day. Plus, having a more structured schedule during the day will also help. This is difficult at only 4 months, but you can have somewhat of a schedule. At four months old, make sure he has a nap in the morning about 3-4 hours after he wakes up for the day and another in the afternoon whenever he gets sleepy enough. This typically works for my son, who sleeps from 7:30 pm until about 4:30-5 am. Every child is different. However, some kind of general schedule throughout the day to give him consistency will (over time) prepare his body and brain for bedtime, allowing him to sleep better. Oh! And teething can also be affecting his sleep. Use Little Teethers or Tylenol for those times when he's in a lot of pain.
K.:)