Here's the process:
1. Create a file the same size as the image you are halftoning.
2. Create 5 layers:
2.a. white, Gray w/ Value = 75, Gray w/ Value = 50, Gray w/ Value = 25, and Black.
2.b. Order the layers so white is the bottom layer, and the layers get progressively darker until you get to the top layer of black.
3. add a layer mask to each layer
4. Open your original gray scale image
5. Open the levels dialog and set the end points to 0 and 64 and click OK
6. convert the resulting image to the halftone pattern of your choice
7. invert the colors of the resulting halftone
8. copy and paste this half tone into the layer mask of the other images black layer.
9. Repeat steps 4 through 8, but this time use a levels range of 64 to 128 and paste the inverted halftone into the layer below the previous. Continue repeating steps 4 through 8, each time incrementing the end points in the levels dialog by 64 and pasting the inverted halftone into the next lower layer.
10. You should now have a 4 color halftone that more accurately represent you original image.