The 4xxx series firmwares are currently factory install only. There are a few H revision units starting to come through which have HW Version 61 and Firmware 4xxx series installed. I'm led to believe these are 40gb / faster processor models and include DAB/DVB. I haven't physically seen one yet (only pictures) so can't comment if that is true or not.
What I can tell you is it will only work on the following model RNS at present:
1T0035680D
1T0035680F
1T0035680G
3T0035680B
3T0035680C
3T0035680D
7F0035680A
7E0035680A
As with the 3xxx series originally only working on C revision units, give it a few months and a version will be released to work on previous versions. I think you will find that all support for both A & B revision Units will dropped same as all support for A revision units was dropped with 3xxx firmware.
So by my reckoning the firmwares will go something like
A Revision units - All firmware up to 284x
B Revision units - All firmware from 110x up to 398x
C Revision units - All firmware from 268x up to 398x currently with support for 4xxx series to come at some point
D Revision units - All firmware from 3xxx onwards
HTH
Carl