Gold 4 after doing placements 6-4 got Silver 4, what the heck????

It's not straight win/loss. It's also KDA, wards placed, creep score, neutral objectives and turrets taken, KP percentage, and so on.

Basically, the game measures how active and effective you are as well to make sure you're not being carried.

Placements will almost always drop you a tier unless you do 10-0 / 9-1 and play perfect, even then it will put you in the fifth division.