As a (perhaps too) general rule, maximum sharpness for a lens is somewhere between 2 and 3 stops below wide open (thus, the f4 45mm Xpan will be at its sharpest around f8-f11).
As Keith has mentioned, diffraction will probably set in around f16. Mind you, it will be so insignificant at f16 and on such a small format to be almost undetectable at f16 and probably even at f22.
Also, the untrained eye will see a greater DOF as sharpness, so you should not fear shooting at f22. But, as Keith (again) mentioned, try it yourself. Use a 10x loupe to have a look at 2 scenes shot at different setting and see if you can tell the difference, and if you can, is this difference a show stopper?