Dishwasher?

It's possible that the screw holding it in place have become loose.
Remove, clean, and replace the screw/knob holding the arm in place.

Look at where it secured in place.
It's possible that the metal have been worn away. I've seen that in an heavily used 15+ years old dishwasher. In that case, you need to...
1 - Find a replacement spray arm.
OR
2 - Get a new dishwasher. But after only 4 years, it's still almost new. Normaly, any dishwasher should last at least 12 to 15 years without repair.

Kuälinar
February 2012