Can you turn codes from action replay ds on and off?

Yes you can. After putting in the Action Replay and then removing it you place your game in the DS. At that point the screen will show a check mark on the bottom right or left of the screen, can't remember which. From there you can choose which cheats to apply or turn off.
No, the Action Replay won't read the R4 as a game, so it won't work. Though depending on the R4 and where you donwload your games from on it, it might come with cheats programmed into it. Just press the y button when the game is highlighted to choose which ones.
But again, only some R4's have this.

Yup, would be stupid if you couldn't.

Yeah, you can turn the action replay cheats on and off whenever you want. You can use them on R4's.

If you don't want to ruin it than take out the action replay. And yes you can tun off the codes by pressing A when it is selected(the dot or check goes away if you do). Honestly I do not know what an R4 is.

Yes.