Here is a quad gene cross I did on a different forum that supports html tables.

http://constrictors.com/forums/showthread.php?t=17097

It was for a specific recessive combo I was asked for but Punnett's are for inheritance possibilities and inheritance happens at the genotype level which is independent of mutation type (as long as we exclude sex linked). So we could consider this a quad co-dominant cross if we just change the key to define the letters differently.

It could use some more polishing but if anyone wants it I've got Excel formulas and a macro to make these html type Punnett squares much more easily than it looks.