For those of you who's newborns (under 3months) sleep through the night or for longer than 3hour stretches. Are you EBF or do you give formula at night?

We've gotten lucky and this past week got ONE 4hr stretch and I was so psyched! We're considering giving her formula when it's bedtime because we read that formula fills up a baby more than breast milk. Thus longer sleep stretches.

LO is 6weeks 2 days.

Thoughts?