At 4 years old it may not be old enough to bloom yet. Don't add any fertilizer as this sometimes causes vegetative growth at the expense of blooms. This includes fertilizing the lawn surrounding it.

At least you are not having to deal with the many seeds it disperses. I know you'd wish it would bloom now ... but you may not be so happy when it starts. I had two of them and the amount of seeds were horrendous. Sure they are easy to pull up the first season after a rain, but who wants to do it? Plus they were in all my neighbors' yards too! I eventually cut them down.

Thanks for the help.

We just cut ours down- it was pretty old and was dying. It was beautiful when in its prime, but like Esh, I hated all those seedlings. Also, it attracted Box Elder Bugs, which in themselves didn't seem to damage anything but they were here by the hundreds. We are not going to replace it with another.