For starters, there are no HD 4:3 standards. All HDTVs are 16:9. So if you want 4:3, you'll have to step it down to standard definition.
Is that what you want?
Create a new sequence that is 4:3 with a PAR of 1.094 (720 x 576 in PAL) or PAR .909 (720 x 480 NTSC)
Place your HD footage on the timeline and scale it to fit. You have a choice of either scaling so that the sides fit, in which case, you will have black tramlines at the top and bottom of the frame - or scale so that the top and bottom fit, in which case you will crop the left and the right vision.