You should measure out about 2" from the left side of the collar, and then measure about halfway down the sleeve. If you draw lines that intersect from those two points, you should get a good placement.
You might want to use chalk to help you see where your art will be. Hold it up and make any adjustments that you feel may be needed. I, personally, might even make a larger logo for the 4XLs.
Hope this helps!