Adobe Illustrator / Vector Trace

1. Prepare an image in Photoshop
2. Image > Adjustments > Desaturate
   
3. Image > Adjustments > Posterize - Levels: 5
4. Duplicate the layer
5. Image > Adjustments > Threshold
6. Create 5 pattern guide layers by changing Threshold Levels
7. Open the Photoshop file in Illustrator
8. Apply Live Trace on each pattern guide
9. Adjust colors