The few arrows you can find around are just for atmosphere / decoration. One person reported being able to equip an arrow, which caused their mage (IIRC) to start jumping up and down.
Depending on how much you have left on the third level, and how difficult the wraiths are, it might be almost as fast to keep going as to backtrack. The forth level is basically a smallish room, and contains a shortcut back up to the first level.
