I think there is a direct bus from Mex City to Tlachichuca, I know there was one back. You could arrive and take the bus (or a taxi). Next day motor up to the hut, one day acclimatization. Day three summit (I started up at 2:15am and was back at the hut at 10:15am) and return to Tlachichuca, maybe even catch a bus that day to Mex City. Day 4 back to an afternoon flight out of Mexico City, the bus leaves very early so perhaps you could make an earlier flight home. Scott is of course right, some people acclimatize faster/slower than others, this schedule allows very little "slop" time. Unforseen problems would result in no summit. If you were really lucky, you could illiminate the acclimitization day and do it in three, but this would allow for hardly any sleep. I think 4 days is very doable, three is also possible without killing yourself.