When you are on the print screen, you should have the option to turn scaling off, or choose no scaling.
According to Wikipedia A4 is a tiny bit longer and a tiny bit narrower than Letter size paper, but only like 6mm narrower, so you should be OK. You may have some clipping of the design if it is too close to the edge.
Most people's printers can't print all the way to the edge. Most have at least a small border called a print margin, unless they are fancy photo ink jet printers. So you should be OK there
