What is so bad about these two moves in the French?

Well, first of all, pawn moves aren't the preferred sort of move in any opening. You should always try to develop your non-pawns as quickly as possible, and this tends to be true all game long (not moving pawns without a good reason).

But anyway, after 4...c4 white plays b3 and or d3 which will force black to capture, which will accelerate white's development (recapturing tends to be better than initiating a capture in the opening... it also tends to be true all game long).

4...b6 doesn't look so bad on the surface. I guess it's bad because you can't develop the queen to b6 which is normal in french advance structures. The basic middlegame idea is to pressure white's d4 pawn in this structure, and apparently black becomes semi-permanently passive after 4...b6

https://www.chess.com/article/view/the-principles-of-the-opening

https://www.youtube.com/watch?v=B-k2fRVYeFg&ab_channel=RemoteChessAcademy

4....b6 is met by 5.d4. Now the black bishop has no real future on b7 because it's blocked by the pawn on d5. Trying to exchange it by 5...Ba6 loses to 6. Bxa6 Nxa6 7.Qa4+. So Black hasn't really gained anything by the move.

4.c4 has an interesting idea, which is to meet d4 with en passant but two problems,

1.this isnt particularly bad for white because while this does weaken the e5 pawn a little, after d4 cxd3  bxd3, white is ready to castle and has potential ng5 threats while you havent even developed a piece

2. white isnt even obligated to play and can just play  b3. taking would just open white's rook file, and if b5, a4! is good for white.

4...b6 is actually a decent move, not the top engine move but the engine underestimates it.  the idea being that black will try to exchange his bad bishop for white's dangerous bishop that will likely go to bd3. In fact. You see this line in the advanced variation of the french 1.e4 e6 2.d4 d5 3.e5 b6?! but the problem with this specific move order is that white hasnt developed his knight yet, so he can anticipate black's plan and instead go for bd3, ne2, 0-0 with f4-f5 push coming next. In your move order, white has settled his knight on the less dangerous f3 square in this position, so his plan is sound.

Yet another advantage is that in the advanced variation move order, white can sometimes give a check on b5 to force bd7 and then retreat to d3 where it wanted to go anyways but now ba6 is not possible, but in your move order bd3 just blocks the d-pawn. BUT just keep in mind that if you want to play it his way, you need to play qd7 first (or sometimes a5) first to avoid hanging a piece via bxa6 nxa6 Qa4!