I think 5 a3 is reasonably safe - after 6 .. Nc6 I guess you can always play 7 e3 but you'd like to play 7 Bg5!? with a nasty permanent pin. You still have to be careful to stop e5 though but eg 7 .. h6 8 Bh4 d5 9 e3 Qe7 10 f4 to stop e5 seems fine for white. 8 .. e5!? is reasonably interesting too.

6 .. Ne8!? stopping any kind of Bg5 ideas and transposing to a main line of the Samisch is meant to be the main line here I think.  You can safely play e4 etc after that at least.

There are similar annoying problems after 4 .. Nc6....
( 5 e4 is maybe better than after 4 .. o-o but it can in fact quite easily transpose to your game.).

Once you've had to give up on quick e4 ideas though you do start to wonder a little why you haven't just played a Samisch in the first place
(Presumably because 4 .. o-o/.. Nc6 are quite rare.).

Hmm, I remember getting the impression from the easy guide to the nimzo - book that black had to take immediate action with ...d5 or ...c5 to act against black`s plan of e4. 

Funny if after 4.f3 0-0, 5.e4 turns out to be !?, rather than !

Certainly the game and some of his notes look pretty good for Black, but he does suggest improvements for White, but most notably at move 5!