File > Project Properties > . . . For 4:3 pick off a Template that DOESN'T have WIDESRCEEN in the template, and for your region. And conversley for 16:9 pick-off a template WITH "WIDESCREEN" in the template.
It wont SAY 4:3 or 16:9 - the only thing that changes is the Pixel Aspect ration. For me in PAL land 4:3 is a PAR of 1.0926 (PAL DV) - For 16:9 it is 1.4568 (PAL DV Widescreen).