I don't know if it will be useful, I'm the sort of person who can cope with things better when I understand them.
So I think you might need a lesson on the physiological changes that happen to baby sleep at around 4 months.
So in the first 3-4 months of baby's life, sleep is passive. As long as all needs are met (calories, warmth, comfort) then baby will sleep. It's like it was in the womb - this period is often called the fourth trimester. At around 4 months old sleep matures. It stops being passive and will never go back to being that.
Sleep develops into an active endeavour - it takes work to get to sleep rather than it being passive. It also develops into sleep cycles, like adult sleep. This is periods of deep sleep and light sleep in repeating cycles. This is new for baby, they were not having cycles of light sleep before. If baby's sleep hygiene is poor then waking up in the light sleep phase of the sleep cycle is easier - which causes frequent night wakes and shorter daytime naps.
Central to good sleep hygiene is to go to sleep where you stay asleep. So if baby goes to sleep in your arms, good sleep hygiene means staying asleep in your arms. If baby is to sleep in the cot, good sleep hygiene means going from awake to asleep in there. It is poor sleep hygiene to go to sleep in your arms and then be put down in the cot already asleep.
That brings me onto settling technique. The way you get baby to sleep becomes of very significant importance post-4-months. You don't mention your settling method - how do you get baby to sleep?
Daytime naps are usually short at this age, 30-45 minutes is normal. Baby will get over tired if there is too long awake between naps. I would aim for 90 minutes awake. That includes settling time, so if it takes you 15 minutes to get baby to sleep (in the cot with dummy) then start doing that 75 minutes after last waking up. This means more naps per day to get better daytime sleep, while naps are short.
Naps lengthen through practice. So for every single nap, try to resettle to help baby link from deep sleep to light sleep back to deep sleep, rather than waking up. To do this you need to sit with baby as he sleeps. Catch him coming out of the deep sleep, before actually waking up. This might show as a small movement - arm or leg moving, face screwing up, shuffling position, this kind of thing. If baby opens eyes or cries then he has actually woken up, so it's too late to resettle.
Try to disturb baby as little as possible when resettling, certainly leave them wherever they are asleep. Try patting or shushing, comfort sucking also helps. Resettles wont be successful every time. Don't worry about this, baby is still learning to deal with sleep cycles. But always try and then it will gradually become more successful. In the mean time while naps are short, keep awake time short to avoid over tiredness.
HTH