With JDK 5 you could also queue up tasks for a thread pool with only one thread. That would let you queue any number of tasks to run in parallel with your main thread, and know that only one of them runs at a time. I can almost imagine that being a real requirement.
I guess you could also start all five and have them synchronize on a shared object for almost the same effect. Sequencing their access to some shared resource is a bit easier to imagine as a requirement.