I am back in the thread to answer Sjim's post...but if would be better if Inn2 gives his own reply.
I give some variations with the "c4" idea in the French Tarrasch: 1.e4 e6 2.d4 d5 3.Nd2 c5 (one "main line", often leading to isolated pawn game) and now
A) 4.Ngf3 ("universal system" ?) Nf6 (in other lines 4...cxd4 ; 4...Nc6 ; 4...a6 the c4 move will not occur because White will play some dxc5 and exd5 moves to give Black an isolated pawn on d5 square...maybe the most flexible a6 is used, like in Gurevich's games ) 5.exd5 (5.e5 is closed main line where c4 is not in Black strategy: attack d4 pawn with c5xd4;Qb6.. ; 5.Bb5+ leads to isolated pawn game after Bd7 or Nc6 example: 5...Bd7 6.Bxd7 Nbxd7 7.exd5 exd5 Carlsson-Potkin,2001 Mueller-Kindermann,Autriche 2002; Rozentalis-Ross,Montreal 2003 or Yemelin-Dolmatov,Novgorod 1999; Kharlov-Gurevich,Istanbul 2003 (Chesspublishing) ) exd5 (5...Nxd5 is one good idea to play without isolated pawn as given for example, in McDonald&Harley's book about the French, game King-Dolmatov,Reykjavik 1990) and now 6.Bb5+ Bd7 and game as given above ; 6.dxc5 Bxc5 7.Nb3 isolated pawn game without c4, maybe one "safe" line for each side of the board ; 6.Ne2 Nc6 7.oo c4 ; 6.c3 c4 ; 6.Bd3 c4 7.Be2
B) 4.exd5 exd5 (4...Qxd5 not in c4 stategy but one other main line) 5.Ngf3 Nf6 transposes to A)
C) 4.c3!? Nf6 (Dom's advice ; if 4...cxd4 there is McDonald/Howell/Crawley's idea to go off-theory: 5.cxd4 dxe4 6.Nxe4 Bb4+ 7.Nc3) 5.exd5 (5.e5 closed main-line ; 5.Bd3 Nc6 6.Ngf3 Qb6 Dom but you can try the over variations 6..a6!? ; 6.cxd4 ; 6.Be7 ; 6.h6) exd5 6.Ngf3 c4
D) 4.dxc5 Bxc5 5.Nb3 leading to isolated pawn game