I've read at couple of places season 4 is scheduled for Feb/March 2011, however it isnt on any upcoming anime lists.
Read somewhere else that Feb is the starting point for production of the new series so it may actually end up being released this summer instead.
The good news is that there definately will be a season 4. Since the voice actors for Saito and Loiuse also provide voices for Shakugan no Shana, the team decided to do Shana first and then when that is finished to then do ZnT.
Though Shakugan no Shana only ended uo with 4 OVAs, I'm not sure if they are going to so the Shana series first or ZnT.
All I know is that I am very impatient for both!