A colleague once showed me that I can create a square and pre-select the 4 corners and then hit a certain icon and all corners become dimensioned and linked with a formula. I just don't remember which I icon to click.
never mind, I figured it out. you click on the corner tool in sketcher and make a box around the rectangle. Then you type the radius in the sketch tools cell. this will constrain all 4 corners and link them in an equation.