When I went to swap the handle around one of these tiny four screws was out. It's so small I can't tell what tool would be used to put it back in. It actually looks like there is a little nipple on the end so it doesn't look like an Allen head 

it's for the drag star clicker. Just set it back in the spring and put the plate back over it. There's small dimples on the plate that when hit by the pin, makes an audible clicking noise when rotating the drag star. 

1 hour ago, iabass8 said:

it's for the drag star clicker. Just set it back in the spring and put the plate back over it. There's small dimples on the plate that when hit by the pin, makes an audible clicking noise when rotating the drag star. 

Yep, and if you lose one, your drag still works, it just doesn't sound near as fancy. LOL

It is a tiny black plastic like disc.  Also, be aware that in the little well under the "dimple" device there is an itty bitty spring.  Whwn cleaning my reels I put a tiny dab of grease over the whole thing to keep it in place.

Like said before, it's not a fastener, but a spring and clicker that works with the ridged, black, plastic disk the goes on top of it in the stack.  those three parts make the clicking noise when adjusting the drag.  Tweezers help my dumb fingers place it on top of the spring.  Then hold that black disk while you put the rest of the handle assembly together.  If you lose it for some reason, it can be ordered from Shimano directly.