The will grow more than half in the flowering stage. Search this forum and you will find this out. Mine grow only 1/3 in veg and 2/3 in flowering. I would trim and switch yesterday.
They will definitely get bigger. I wouldn't chop the top, especially since they are that tall and you are wanting to flower. If you are using 1KW of light, that might not be a bad idea, but anything less and you will be waiting a lot longer than two days for it to recover. You can use the LST method when they are that tall and gradually tie the branches down, starting from the bottom. Basically, get the plant to grow at an angle and you will not have to worry about it hitting the light or getting to tall. Tying it down is definitely better than chopping.
i always expect my plants to double in size from the time i start flowering. so yes, you're going to have to figure something out if they are 3 ft tall already before flowering and you only have 4 ft of room. as spike420 mentioned, LST'ing might be your best option.