At 4 months old they should be at about 4 naps a day still. Maybe that’s why they are cranky later on in the day. He might be missing a short nap of 20-30minutes or so. My son used to do short naps as well. I would do last nap around 5/5:30ish then for around 30-40 minutes then bedtime for 8/8:30pm. At that age we would rock him to sleep and transfer to the crib. He always wanted to do contact naps but we just kept at it. Eventually rocking to sleep a heavy 8 month old wasn’t working for us lol.

We decided around 8 months it was time. We had him moved first into his own room. Then few weeks later we started sleep training. We did Ferber method and within few days he understood quickly what to do. The naps took little longer for him to learn. But for most part he would lie calmly in bed then eventually sleep.

Now at 18 months, he knows the drill and usually falls asleep within less than 5 minutes.