4.3 is fine
to downgrade you would need to hack 4.3 first
might just as well stay there once you did it
edit: overlooked the nand backup part
that changes it a little
if you have one of the 4.3 hacking games simply rehack as above
if you don't, restore 4.0
there are ways to install netflix there too afaik
but I prefer updating the hacks to survive 4.3 and update to 4.3 again
(eg by installing in this order ios58, newest hbc,, some cios to 248, 4.3, everything else you want)