I do agree with JeremyL and green4eva – The funivia and jeep make for a very memorable experience and even if unable to trek, standing at 3000mt on Etna is exhilarating!
The Etna funivia posts warnings for those with asthma or heart conditions.
Since afternoon clouds roll in, morning excursions are ideal. Only visit Etna on a good weather day to optimize your views.
Even in summer, Etna’s summit could be cold and windy, so always dress in layers – mac, warm socks, very sturdy shoes, scarf (protection from fumes), sunglasses, and sunscreen.
Group bus excursions are very good values, significantly less expensive per person than private tours.
Four hours is actually a fair amount of time for the funivia, jeep and a trek.
However if you weren’t planning on that tour, four hours at Rifugio Sapienza might feel long, even if you were to walk the Silvestri Craters and enjoy lunch.
SAIS also runs a tour from Taormina.
http://www.saistours.com/downloads/EscursioniBrochureItinerari.pdf
Many of the private excursions include stops at a winery, or tours of Taormina itself.
http://www.sicily-tour.net/tour-etna/index.htm
http://www.legendarysicily.it/escursioni-naturalistiche-inglese.php
http://www.bluestonesicily.com/?page=our_news&id=13&title=Daily+tours+and+excursions%20-%20Daily+tours+of+Sicily
http://www.sicilylimousineservice.com/sicily/Mount_Etna_Tour.asp
http://www.sicilytourguides.net/Mt.Etna_Taormina_tour.htm
If you prefer to solely tour Etna, you might consider taking Interbus to Catania or Catania’s airport, where you would find some excellent ‘Etna’ tours….
http://www.etnaexperience.com/english/excursions/Morning-Experience.html
http://www.etnasicilytouring.com/en/excursion/etna-morning
Best Wishes!
Vagabonda