Thanks for writing. If it's only four years old, your tulip tree may not be mature enough to bloom --- these trees can take a number of years before they bloom for the first time. Also: Be sure that it's in full sun (if it's shaded much by other trees it may not bloom at all). Other than that, all I can really recommend is patience. And keeping a sharp eye out in spring; because the blooms are green, they blend in with the leaves so well. Some people who have tulip trees in their yards never even notice the flowers!
---Justin, Senior Garden Editor, BHG.com