Illustrator would be the better option as it is a vector (lines and fills) program. Photoshop would work just as well, but since it is raster (dots and pixels) it will be more dependent on the quality or resolution you set when you create your file. However, Illustrator won't necessarily give you a better result. Illustrator will simply create a crisp edge no matter the size you enlarge to. This is because there is no such thing as pixelation in a vector image. Photoshop has some really cool effects and tools that you can't do in Illustrator though. For an easy answer, I rarely design anything in one or the other because I always use both and jump back and forth. If you use File -> Place to move parts from one program to another, they remain in their native formats (vector or raster).
In Photoshop you can simply type in "4 in" and it will resize to 4 inches, just as you can type in "4 mm" and it will resize to 4 millimeters. Also you can change your units like
this.