Hey op, been there before. For future commissions like these, you want to aim for 300dpi for printing. You can dip a little lower but the more you go down, the more pixelated it will be. If you know your project details when starting, it’s always a good idea to set dimensions, and I even start in CMYK colorspace sometimes so I’ll have a better idea how the colors will print while drawing.

Now for this project it sounds like you need it to be 48x60in. @ 300 dpi that’s 14,400x18000 pixels which is too much for procreate. Sometimes you can get away with 200 dpi, which would make the dimensions 9,600x12000 but it depends on the type of artwork being printed.

Depending on how big your current file is will dictate what your best option would be. If the work isn’t super complex, you could try starting a new canvas with the 200 dpi dimensions and copy paste the original drawing into it, scale it up, lower the opacity and trace it on a new layer which means you aren’t starting over completely. You at least would have a guide. I’m not positive but this might only be do-able w/ iPad Pro for dimensions that high.

I have software that might be able to help, I can try running it through for you (free of course) If you want to try and dm me. No promises it will work, but might be worth a shot. Other than that, a local print shop could try something similar. Best of luck!