We have a non-USTA league that plays a 3S/4D format with prosets (8 for S, 10 for D). Players can play both in the same morning. "Promotion" is done on a team basis and relegation style. Top two teams move up, bottom two move down.
Have to say, it's highly successful around here...and infinitely more laid back than USTA. There is no formal "league fee," rather there's a relatively nominal team fee and most teams include the season's worth of balls before dividing it up. Home (club, neighborhood or reservable public court) team provides courts. This year, I paid $13 to play two, 7-week seasons, split between Fall and Spring. There are fairly liberal "substitution" policies and many teams run with a thinner roster since they can "sub" in a player from a lower division. I have been able to play in every week's lineup that my personal schedule allowed.
And, because the it's the "team's" placement, individual players can range all over the NTRP spectrum or not even have a rating at all. There are some rules about having to play "in order of strength," and after having played for several years, the vast majority of teams play it that way. Our team ranges from a 4.5 to 3.5s and I've had the pleasure to play with them all at some point or another.
It's so fun...even our club pros play in it...while they often shun USTA (or can't because there's not a league/team for them in our area).