If you win 4 games in a row, you will be about 100 SR higher, so your games will be about 100 SR harder, on average. There is a small performance modifier, but maybe that would mean you gain 120 SR in four games instead of 100. This difference is going to be too small for you to feel.
Yes.
Doing this will cause you to gain less SR/MMR, which will hurt you overall.
To see how things actually work, read How Competitive Skill Rating Works (Season 13), especially the summary.