I'm sure EXT.NET team is also excited of the EXT JS 4 new capabilities, and they do their best (also because their business is based on this) to deliver when it is ready.
We are working on V2, just it is early to say about any time frame.
Please note that ExtJS 4 has absolutely different API (data, grid, tree, another themes (there are no Slate and Gray), many ux are not updated for ExtJS4) therefore can be many breakable changes
As well, the Ext.NET v2 build (includes ExtJS 4) is progressing nicely. We should have a preview build available to Premium Members sometime shortly after v1.0 final release.