No, you don't have to declare charges at the start of the turn, but I'm not sure if the geometry of what you're considering is easy.
If you're a Cav unit (presumably in "Waves") and you're only 4 BW from an enemy square, then it might be tough to get a friendly unit between you and that square. An infantry unit probably doesn't have the movement rate to get there (since you're nearly 2BW deep, and he'd have to move through you and then in-between you), unless by some chance he was already nearby, say, just to the left or right. You might have a friendly Cav unit with enough movement to get in the gap, but, that might require a DISC test for interpenetration, so it's not guaranteed.
And then there's the question of whether you can wheel enough, with a friendly now sitting directly in front of you, to legally charge somebody else without interpenetrating. You've only got the 2BW maximum wheel, and then the charge move can go straight ahead.
You might think: "Well, I could just get a bit of a friendly unit in there, blocking the charge path to the square; it doesn't have to be the whole thing...." But it has to be fairly substantial; enough to block any possible wheel-and-charge path to that square.
But: if you can legally surmount all these obstacles, then Yes, you pull off the maneuver you're talking about and "cover" the square with another friendly while your cavalry charges somebody else.