Hi there,
You could try maybe 1hr 45mins in the morning to see if his nap will be longer... but honestly, 4 months is a yucky age (4 month sleep regression, big growth spurt), and short naps are common. If he's only now learning how to self soothe, it might be a matter of him not yet knowing how to put himself back to sleep after a short nap. I used to leave mine in the cot for another 30mins if her nap was only 30mins and sometimes she would put herself back to sleep. If he's not crying, you could leave him but if he's crying and its not a mantra cry, it's probably best to use shh/pat as holding him to extend the nap may lead to him getting used to that method.
The av A is 2 hrs but if he's only had a short sleep, it's probably best to adjust the next A by 15mins or so. You may end up having 4 short naps or if he's fighting the last nap, make bedtime earlier.
Hope that helps a little