My wife and I took a break from it for awhile but we eventually finished with the fourth season. I saw it long ago but it was her first time. I will say this is the first season to really be overshadowed by previous ones.
I'd say the best of the whole season is when all the adults go to prison (thanks Cartman!) and the town is run by the kids of South Park. Doesn't taken even 2 weeks until South Park is dystopian beyond belief.
I'd say the reason the season is rather weak is I suppose the show creators thought everyone loved Cartman so much that he would have ALL the good moments. Like starting a practice of being the "tooth fairy". Poor Butters...
Even starting his own faux-church.
And going to fat camp. He should of expected the trick of an ice cream truck in the middle of the night in the middle of nowhere.