doug 2k is correct.. i just installed a color lcd assembly swap and that same lil bugger came off my logic board. the location of this piece is hard to explain but doug did a pretty good job.. look on the logic board, to the right of the rear camera connection, there is a little metal piece of the EMI cover that looks like a Y but rounded, that little piece that fell off went under there..
i'm not exactly sure if the piece is mandatory but my phone is functioning without it..
if you look at step 13 here, (iPhone 4 Logic Board Replacement) your "screw" insert would belong beneath where the yellow circle is..
i dunno how one would repair this part... mayyybe using some krazy glue gel (?) ---> which i wouldnt recommend because that would be apparent to a tech if you ever chose to warranty the device.
sorry so wordy..