Scar tissue will hardly accumulate enough over biweekly injections for a measly 12 weeks. Certainly not enough to feel resistance.
And the skin absolutely wont thicken. Scar tissue doesnt occur in the skin.
I doubt it would be painful to rotate quads for biweekly injections.
I DO think, it is not worth the risk.
So I think you got it backwards.
4 sites is fine for biweekly injections. Keep the injections to 1ml or less and you'll be better off.