A 4 star team average is still good. Don't go selling players just because they have lost a star, if they were performing well the previous Season hang onto them.
Also throughout the Season give an extra training every so often to the players you want to keep, and make sure the older ones get in as many matches as possible to help keep them with you longer.
I rarely buy in the first few days, but I look at the TL the rest of the Season and if I find a really good player, even on the last day of the Season, I will buy him. I save a lot of Tokens from auctions and spend them on training, even on a team I don't buy tokens for, I never replace as many as you do, usually just a handful.