Crying after nap...

Nichola
So this is a bizarre one...
My little boy naps 4 times a day for around 45 minutes after a wakeful period of 1.5 to 1.75 hours. He turned 4 months on 30 August.
His first 3 naps are great, he self soothes and goes down well and wakes up well.
Nap 4 he self soothes and goes down well but when he wakes up he screams. And I mean screams to the point he is inconsolable. 
Me and my husband know he will do this now every night.
My routine seems fine, his ability to sooth is fine so why could be be crying like this on his final nap?
It's a strange one. I'm not sure if it's just a thing he does.
Any input would be valued!