georgiad, good work! 4lbs is as far as you can go for a week.
mumtotwo, squats after the 100th mark are more of a matter of concentration and deep breathing. The thighs do not tire so much as the whole body does. It's like a zen-meditation session, only with exercise and not stillness.
As I said, breathing is super important. I breathe out just as I hit the bottom of the squat and then explode and breathe in at the top.
Also, I do not do them every day with such high numbers. Maybe every other day or so...