To answer your question, #4 is Ryan Griffin. But as stated in the quote below, the NFL restricts what numbers players of all positions can wear.
QB's = #'s 1-19
WR's = #'s 10-19 & 80-89
DB's = #'s 20-49
And so on and so forth. That's why Reggie unsuccessfully petitioned, and couldn't use his college number of #5.
That's why teams rarely ever actually retire numbers, because there are simply not enough numbers to go around by retiring a few players at the same position. That's why teams "retire numbers but are still given out to active players".
Bingo.