4 month old waking

My 4 month DS has been waking at around 9/9.30pm each night for the last couple of weeks. His bedtime is around 6.30pm and he wakes at different times each night. I give him a BF then he sleeps until around 2.30am and will sleep again until 7am. It's really weird cause he used to sleep until around 12.30/1am for a few weeks then something changed and I can't figure out what. I thought he might be too hot so I've tried to keep him cooler in the first part of the night. I though maybe it was because he was going to sleep when it is bright then waking when it is dark so tried a night light, but I think that made it worse cause he then woke more throughout the night. He's really not happy when he wakes and isn't getting a good block of sleep because of it. Anyone had this happen???

Up until about 12 months bubs will go through different stages of waking, then sleeping through, etc. Last week my bub started waking through the night again, but this week everything is all back to normal.
It's completely normal, every baby does it and you may not have changed a thing. It could also be a growth spurt, they have one at 4 months and another at 6 months.
Hope he goes back to his normal routine soon for you.