how can i figure out which of the greyscales the empeg supports?

Draw a picture in logoeditor using each of the four colors. Copy that image to the clipboard and paste it into your image-editing software as a new image.

Your image-editing software should have the ability to create a palette based on the colors in this new image. If you can't create a palette of four colors, create one with 16 colors where 13 of the colors are just black.

Then use this palette for all of your color reduction work.