Higher overall game winrates = high mmr?

So basically if you have high winrate you would have a high mmr?

Some of my friend has 50%> winrate and they have 4k+ mmr, 50% winrate = 3.5k mmr,
below 50% winrate = 3k mmr below?

Not necessarily. I believe a lot of the MMR calibration came from other stats including KDA ratios. Obviously it helps if you have a higher win rate, as you're generally doing well, but as you pointed out, it isn't the only decisive factor.

I thought I'd read that kda did not effect or only had an extremely minor effect. If it did in anyway have much of an effect it would be highly prejudicial against support players and would encourage selfish play. So I hope you're wrong, but I'm not 100% about it.

In answer to the OP no overall win rate doesn't have an effect on starting mmr. The 10 callibration games supposedly did. But I think the secret mmr had more of an effect.