3weeks old sleeping longer than 4hrs at night

Tonight, my LO has been sleeping for 4hrs at night first time. He has had rough nights with gas and constipation from supplement formula and from last night I have been giving him my expressed breast milk. And he is not fussy and sleeping so well !!! He only moves when his pacifier falls off and when I put it back in his mouth, he falls back asleep. Should I wake him up ? Or leave him sleep longer??