F4 you want to play H5, Bishop C4, Knight H8, Knight F5, after you push the caro pawn like usual on move 2. The idea is if they push the pawn to attack your bishop you let them take opening you rook. Then you have bishop C5 mate or close to it with the rook and bishop or queen checking, it's just all kinds of nasty stuff. look at it with an engine.
@@KeepChessSimple Yea G5 pawn break is useful in those position where they have the F pawn out to, to break up the 3 pawns apart. G5 is best in those positions sometime.
You can treat early f4 against Caro-Kann as very bad French defense for white. The most frequent scenario is something like 1. e4 c6 2. f4 d5 3. e5 Nh6 4. Nf3 e6 5. c3 c5 6. d4 Nc6 7. Be3 Nf5 8. Bf2 cxd4 (8... Qb6 is also possible, but it transposes to the given line) 9. Nxd4 Nfxd4 10. cxd4 Qb6 11. Qd2 Bd7, and black has good French-type position: black lightsquare bishop is not so ugly there, and certainly it will have some activity on the board, and even in the worst case of scenario you will be able to exchange it for knight.
