there is a program called wine.
currently doesn't work with windower 4 though.
least i never got it to work.
my sister uses wine to play on mac laptop.
I sent him a message, but to answer the question publicly for those who seem to have it wrong. VMWARE Fusion. its easy to figure out, however its more diff to get the graphics looking decent more than anything else given ffxi's old af.
VMware runs it pretty decent and can operate windows 10. There's an option to wrap the apps and have them run as apple apps instead of inside a window. It works with windower4.
There'll be a few issues with some shadowing tho and the fps will be a bit down.
Best option is to just run a full windows partition.