In my book i cover both 4...Nf6 and 4...a6. The problem with 4..a6 is a line that both Watson and Tzermiadianos fail to notice.
After
4...a6 5.exd5 exd5 6.dxc5 (6.Be2 Nf6!? is not a problem for Black. Of course i analyse also this possibility in detail)
6…Bxc5 7.Nb3 Bb6 and now
8.Bd3! is best as was used by the Tarrasch expert GM Tiviakov.White has a clear plan of O-O+Re1+Be3
In this case i discovered only one variation where Black gets equal chances:
8…Qe7+! (Watson gives only 8…Nc6 9.Ο-Ο Nge7 10.c3 Bg4= in his Dangerous Weapons book,but 10.Re1! with the idea 11.Be3 is just better for White)
9.Qe2 Nbc6 10.Qxe7 (else …Qxe2)
10…Ngxe7 11.Ο-Ο (idea: Re1+Be3)
11…Bg4! 12.Re1 Bxf3 gxf3= and the bad pawn-structure balances the bishop pair.
But the problem comes from Tzermiadianos proposal:
5.dxc5 Bxc5 6.Bd3 Nc6 7.Ο-Ο
(to tell you the truth Tzermiadianos doesn't cover 4...a6 in his book but you can easily transpose to his suggestion after 3.Nd2 a6)
where Black must play
7…Nge7! (every other move is inferior according to IMTzermiadianos and IM Rendle in his chess.com lectures)
and Tzermiadianos now continues his analysis:
8.c3 (8.a3 Ο-Ο 9.b4 Ba7 10.Bb2 Ng6=, ΙΜ Rendle)
8…Ο-Ο 9.Qe2 Qc7!? 10.Nb3 Bd6 11.Rd1 Ad7 12.Be3 where he thinks that White is better but...
12…Ne5 13.Nxe5 Bxe5 14.g3 Bc6 15.Bc5 Re8 and the position is balanced
My question is (i express it also in the book) :
"What happens after the logical
8.exd5?"
It is clear now that after 8...exd5 White is better he can improve Watson's variation. So only
8...Nxd5 remains, but now
9.Ne4! Be7 10.c4 is better for White.
After all 4...a6 doesn't equalise but remains playable and the specialists haven't realises yet the way to get the advantage for White, so i don't expect the opponents of my readers to achieve this. But, i was honest to write all these and offer the alternative 4...Nf6 where i provide an equalising solution there.
As i always say in my book reviews and critics, i am not in the level of these guys that write these books (both Watson and Tzermiadianos are far better writers, players and analysts than me) but such details do not escape from me. This means that today's chess writers don't put too much effort to their job. But this is just a personal complaint
And finally to answer Stigma's question. I think that in both GM level and below that this variation can be played for the win. If you see the main line of my book you can see Black putting his Rooks on e5 and e8, Qd7,Ne4,Bb6 and he can do also Nd6-Nc4 and Nfe4! I call this a "winning formation!"