Hi,
Four hours seems slightly optimistic, I would think 5-6 is more likely, but 4 is certainly the right ballpark. The shared taxi depot in Tashkent where you go to get shared taxis to the Fergana Valley is the WORST such place I have ever been in my life - the only thing I can compare it to is Lagos. There is an unbelievable amount of hassle at the depot, especially when you stand out as a foreign tourist. It took me well over an hour to even begin looking for shared taxis here, because for 45 minutes I quite literally had over 20 men following me around, taking every step with me and screaming at me 20 different things. In the end, I organized a seat in an efficient taxi at a very reasonable price (I was going to Kokand, incidentally), but it took a lot of time and patience to get it done. Be prepared for some hassle and time-wasting at the depot!
If you only have 2 days to visit the Fergana valley, I think you're picking the wrong places to visit. Fergana City is a new (19th century Russian era) town, with little of interest in it. Margilon has an interesting silk factory and a nice bi-weekly market, but as a whole, I don't think it's the most interesting town in the region. I'd recommend Kokand (there's are some nice pre-Russian ruins in town, and the town has a very calm, traditional atmosphere) and/or Andijan (very little of architectual interest, sadly, but a nice atmosphere in town and a fantastic Sunday market - if you're planning on one of your two days being a Sunday, I'd definitely reccommend Andijan).
Just my thoughts.
Giora