The Nikon D400.......

Will the TC-20III work with the older 300mm f4 ED, i know the newer TC-14III doesn't (or at least Nikon states it will not work with it)

Yes it will, but Nikon say:-
Autofocus is available only with cameras that offer f/8 support. These include the D4/D4S, Df, D800/D800E, D810, D600/ D610, D750 and D7100.
 
I have one and it does although the IQ suffers - I tend to use the TC20Elll with the Nikon 300mm f2.8 VR and the TC14Ell with the f4

The lll will be better IQ than the ll on either of those lenses ;)
 
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