Is this even an issue? Users will be able to set whether their video driver stretches or clips 4:3 fullscreen software in their display settings.
Not all video cards support clipping (aka pillarboxing) in the drivers. My laptop does, and so I have no problems with 4:3 games, but my desktop computer doesn't, so I need the software to do the clipping for me. If it isn't supported in the software, then it'll be stretched on my computer in fullscreen mode.
Eventually I'll upgrade my system, and when I do, I'll look for a card that supports pillarboxing. But it'll be a while before that happens since money is a little tight for me right now.
@MGE Developer, you could always give the user a choice. You can have the program default to using Max2D viewports, but if the user is having problems with the graphics, then they can set an option that'll draw black rectangles instead.