The MMR does, yes. Last season it was some weird formula like (current MMR + 1300) / 2. I don't remember what exactly it was, but basically if you were moderately high elo and your MMR was even or low for your division your placements would be against significantly lower opponents, and if your MMR was high for your division it would more or less be the same division that you were playing before
It basically made it easy for low elo to place a little higher, i.e. a Bronze 2 could play his placements but essentially start with a bonus 200~ MMR, but made high elo players drop a couple ranks even if they won 9/10 games because you couldn't make up that much MMR decay just from placements. Diamonds dropped to Plat, Challengers dropped to Diamond and sometimes high Plat, etc. It just makes it so you have to reacquire your rank for the new season.
tl;dr Yes, you'll go into Season 5 with much lower MMR, but at Silver 2 getting 10-15 a win is probably like Silver 4 MMR, so you'll be able to stick around high Silver unless you tank your placements. If you think you're on tilt and are going to keep dropping just wait because you'll get a little freebie MMR bump for being low elo.