I had quadruplets in a full household, so it can definitely go past the limit. I'm aware of those reports igzor was talking about though.

In one of my recent games, with only two spaces in the household and planning for twins, my pregnant sim had quads!!!

I just recently installed Woohooer, the father was one of triplets, the mother had the fertility reward, and I'd allowed overstuffed households, apparently a dangerous combo!

Odd thing was that twins was only @ 300%, trips and quads percentages were so low I didn't even consider it happening.
I've had it be over 1000% for twins and still only gotten a single child.

Sadly, with quads not a thing in the game, they didn't get any baby moodlets at all.

But the grandparents instantly got their 5 grandchildren wish as their son's simbot had joined the household prior.