The difference between 4-6 weeks and 16 weeks can be monumental. It's not a similar comparison to make.
By 16 weeks, you've seen her her at least 15-20 times.
Like Rollo has said before, when someone makes you wait that long, it usually isn't worth it.
I've been one to wait before, and I agree... unless it was a virgin.
Even then, it's only somewhat justifiable if you are adequately spinning plates.
I wouldn't even consider myself "dating" someone if no sex had happened yet. At 4 months, I'd just consider her Plate Z and someone to spend time with, not my first option.