Longer Naps

I just transitioned my almost 4 month old to napping on his own during the day (he would only nap in my arms or would scream bloody murder when I put him down). The transition went well but he will only sleep about 20 minutes. He sleeps great at night about 8-12 hours straight. My current nap routine is similar to night time just shorter. Diaper change, white noise, dark room and sleep sack. I hold him until he is sleepy but awake and I stay until he drifts off into lala land. Any tips to get him to nap longer? Is it developmental or just his personality? He used to nap 2 hours in my arms. Thanks!