It will still be possible to use TinyMCE 3.5 in Moodle. The editors are now plugable and users can now select their preferred editor when more than one is installed. Atto will be the default in new installations, but this will be configurable by admins.
The creation of plugins for Atto is (reported to be) easier than TinyMCE. There is some information on creating plugins at http://docs.moodle.org/dev/Atto#Atto_Plugins but I suspect there is more detail needed. We need a pioneer outside HQ to give this a go and help improve that.
By the way, if we'd upgraded to TinyMCE 4, you would have had to rewrite your plugins as they have changed their API.
Dawn: If it's not faster than the current editor, I'll eat Damyon's hat.