Two things I noticed that took care of the problem and got him back to taking full naps during the day and sleeping through the night again. (hintmama.com)
I had one twin that took full naps and one who took 45 minute naps (waking up angry) until he started walking. (babywisemom.com)
The 4th nap is hit and miss or whether he will sleep 45 min or take a full nap. (babywisemom.com)