I assume you mean that the top of the cabinets are not even with the ceiling?
You cannot fill the entire space with one molding as you already understand. What you need to do on this is to put a filler strip cut from the three inches to the four inches that matches the cabinet color as close as possible. Once matched it should be secured tightly to the top of the frame of the cabinet with screws. Pre-drill holes into the cabinet tops and counter sink holes for screws. Once the filler is installed tightly against the cabinet you then can proceed to install the crown molding as though no space exists. You want to use small crown moldings in this case as the larger it is the easier it is to see that the ceiling is out of plumb with the upper cabinets. I would not use anything larger then a 2" molding along the top of the ceiling. This gives you about 2.5 inches of clearance to the top of the doors or so. Be sure to match the color of the crown molding to the cabinet color so it blends into the cabinet and not become the focal point of the upper cabinet. Doing it this way will hide the imperfection of the uneven ceilings above. If you were to finish in different color this would bring your eyes to the difference and you would see the unevenness of the ceiling area.
Wow...I must say that the response from Mr or Mrs Woodbridge Environmental is exceptional and completely thorough....there is nothing to add to it except to say Happy New Year!
Thank you so much. It was very nice of you to take the time to help me! It solved my problem and it was easy to understand. Thanks again. Barbara.