4 month sleep pattern

my son is coming up to 4 months old he weighs around the 15lbs mark. He sleeps consistently 10pm till 7/8am... some days he'll have only 40 minutes nap, broken into 20 minutes ish at a time. However sometimes he'll sleep for 5 hours straight in the day and still do 9/10 hours at night. 

Is this "normal" do you think? 

i will speak to the health visitor tomorrow but I'm curious.