Thanks for the interesting replies.
4.Nc3 b5 5.cxb5 axb5 6.Nxb5 Ba6 7.Nc3 is indeed a Benko proper, but that's fine with me. Black has avoided some sidelines this way.
I believe most Benko players prefer playing against the main lines, even if those are more challenging than say 5.b6 in the Benko move order.
@TalJechin:
Thank you for pointing out that Martens idea with e5 and Bd6. This line looks a bit artificial, but really could be worth a try.
If Black can play e5, Bd6, Bc7, Ba5 and d6, before White gets anything going on the kingside, Black has some kind of Czech Benoni without the "bad bishop".
Furthermore this idea more or less rules out Avrukh's line with 6.a5. That pawn would simply come under fire.
I quite like this, but I'll have to check if White can take advantage of this rather slow set-up by Black.
However, Milov's 4...e6 5.Nc3 Bd6 looks like the most flexible way of dealing with 4.a4.
Fascinating stuff.
I hope we can collect even more ideas.
LeeRoth mentioned a transposition to the Modern Benoni. That's interesting, too. Are there any lines where the early inclusion of a6 and a4 is bad for Black?
Obviously, Black cannot play the manoeuvre Na6-Nc7 anymore.