My bet is that if there is anything you can do to increase sunlight for the plant, do it (trim trees, bushes, etc.). Crapemyrtles like a lot of sun - my 'Centennial Spirit' is in full sun and blooms its buttinski off with adequate watering. I have a 'Zuni' that has been slowly overgrown with an ash tree (tree seriously borer bitten but amazingly healthy looking) and it has begun to decrease in bloom production. Do some tree or shrub trimming or move your crapemyrtle to a sunnier spot in SPRING.
hortster

I really don't have any other place to move the crape myrtle to. My neighbor has a tree on their property that blocks some sun, but I cannot cut or trim any limbs down as they are on my neighbor's property. Do you think it will do okay with 4 hours of sun? How tall is yours in full sun?

Your Crape Myrtle could reach about 20, if not killed back by a cold winter. I fear that it might be a bit lanky and leggy since it will not be grown in the full sun. Flowering might be weak, as well. But the most troubling problem will be that of powdery mildew.

You see, Centennial Spirit is not one of the hybrids (L. indica x faureii) so popular in today's landscapes. These hybrids bring wonderful attributes to this genus of plants...gorgeous fall color, high disease resistance, and beautiful exfoliating bark. Centennial Spirit is a pure Lagerstroemia indica. Some sites report powdery mildew resistance as fair (which really means terrible), while others list it as good (which means fair).

I fear that your crape will be even more susceptible without the full, blazing sun.