Modern Defence

d5 is not good :P c5 is better

if you don't want to take the pawn then you can play e5 and blacks bishop are locked in

After 4. e5 f6 is a legal move, 5. f4 Nh6 6. Nf3 Bg4 is a suped up Gurgenidze variation.

After 4. e5 f6, it can quickly transpose to a french/caro-kann. Just a quick example:

After E5 the main move is C5

I am not sold on the pawn sacrifice after 4...c5 5. dxc5 d4 6. Nb5 Nc6 7. Nf3 Bg4 8. Nbxd4. (a) 8. Bxf3 Nxf3 9. Qxd1+ Rd8+ 10. Ke1 Nxe5 11. Bxe5 c3 is a pawn down endgame that only White can win. (b)  8. Nxd4 Nxd4 9. Bxf3 Qxd8+ 10. Rd8 gxf3 11. Bxe5 is the computer's suggestion, but Black still has to round up the pawn and White's queenside majority is an asset in this endgame as well. (c) 8...Bxe5 9. Nxc6 Qxd1+ 10. Kxd1 bxc6 is another pawn down endgame. (d) 8...Qa5 was played in Svidler- Chernyshov Rapid 2003 and looked rather convincing for White.