I have two planters that are 3 feet, nine inches by 3 feet, five inches. (The planter is square.) I was wondering what types of small trees I coult plant in these planters. Any ideas?

Arborvitae or another type of evergreen would work well. These trees seem to grow to fit their pot. They will never reach mature height, but they are still very pretty. Another tree that works well in containers is a Japanese maple, crepe myrtle, and citrus trees. Bamboo also works as a container plant. Oh, yea, don't forget about the rose. In a nice sized container it would do well.
Before you make any decisions, though, make sure the tree is suitable for your area and zone.