Maybe our specialist Inn2 can reflect on the pro's and cons of 4.a3 versus 4.f3 ?
In Indian Systems by Pachman, reworked by BH Wood around 1970, I find indeed the move order 4.a3 Bxc3+ 5.bxc3 c5 6.f3 d5 7.cxd5 Nxd5 8.dxc5 transposing.

How about 4. Qb3, much underrated by Nimzo players and not nearly as bad as the books say.

I play this and the defered Saemisch depending on who I am up against.

I have had very positive results with this and the 4. Qb3 variation in particular.

I agree. I understood that it was meant as an improvement compared to 9.Qc2.

Especially for Inn2:

Miranda Teixeira,S - MNb [E25]
EM/O/109, 2002

1.d4 Pf6 2.c4 e6 3.Pc3 Lb4 4.f3 d5 5.a3 Lxc3+ 6.bxc3 c5 7.cxd5 Pxd5 8.dxc5 f5 9.Dd3 f4 10.e4 Dh4+ 11.Kd1 Pe7 12.Dd6 0-0 13.g3 fxg3 14.Dxg3 Df6 15.Ld2 e5 16.Lc4+ Le6 17.Lg5 Df7 18.Dxe5 Lxc4 19.Dxe7 Dh5 20.Ld2 Pc6 21.Dg5 De8 22.Lf4 Td8+ 23.Ke1 Td3 24.Dg3 De7 25.Tb1 Dxc5 26.Pe2 Td7 27.Dg4 Tdf7 28.Dg5 Dxa3 29.Lc1 Da4 0-1

In fact one might consider 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.g3 c5 5.Nf3 cxd4 6.Nxd4 also a line of the English.
Eg 1.d4 Nf6 2.c4 e6 3.g3 c5 4.Nf3 cxd4 5.Nxd4 Bb4+ 6.Nc3 or 1.c4 c5 2.Nf3 Nf6 3.d4 cxd4 4.Nxd4 e6 5.Nc3 Bb4 6.g3.
I prefer to call these lines the Anti-Benoni though.

Concerning the Tromp: some respectable gourmets have swallowed this junkfood ....

Well, it is true that the Catalan avoids the Nimzo, but it does involve committing White to lines of the English (1.d4 Nf6 2.c4 e6 3.g3 c5 4.Nf3) or the Fianchetto Benoni (4.d5) with which he may not be entirely comfortable.
As for the Tromp, well, it is not worth switching away from 1.e4 to play such junk ... 

I recommend 4. f3. This was Shirov's specialty back when he used to play 1. d4 and he still uses it occasionally when preparing for specific opponents who play the nimzo. From your handle/nickname this variation should be perfect.

ALternatively 4. Qc2 is good too. After 4....0-0 there's the new fad 5. e4! and after 4... c5 5. dxc5 you always get reasonable attacking chances (look for Kasparov-chuchelov '03 for a nice example). 4... d5 can be as mind boggling as the botvinnik semi-slav (do i remember a book line where White sacs TWO rooks??  ). I feel the real problem (believe it or not) is 4... Nc6, going to a Black Knight's Tango which is solid for Black.

4. Bg5 can lead to extremely closed positions (probably the most "closed" in the whole Nimzo?). Hence it is not suitable for a "dragonneer".

*Takes deep breath*

After playing 1. e4 for quite some time, I'm toying with the idea of either switching to 1. d4, or adopting both. In trying to prepare 1. d4, I have had the most difficultly finding comfortable lines against the Slav, Nimzo-Indian, and KID.

My question is (in this forum), what line should I adopt against the Nimzo. Preferably I would like something that is both sound and tactical and while an evaluation of "unclear" doesn't bother me, I don't want a line where everywhere I look i just see "=" after 10-15 moves. I've discounted the Rubinstein simply because black has too many (in my opinion) solid replies. I've toyed with the Classical and it looks decent. As I like the Bg5 Semi-Slav lines and the Orthodox QGD lines for white, I've thought about 4. Bg5, though admittedly I don't know much about the theory or the praxis.

Any help is appreciated!

Thanks,

Mitch