Well we have 3 starters coming back (Hawk/Barnett/Poppinga), one MLB in Abdul Hodge, and one "LB" (although he played only on ST) Tracy White coming back.
We do need one additional back-up LB, since it looks unlikely that we'd resign Ben Taylor.
From my count, we drafted 2 LBs, and signed 2 more that went undrafted. From what I gather (interview of Winston Moss and Mike Stock), one of the drafted LBs (Korey Hall) will be listed as a FB, so in fact we got 3 LBs instead of 4.
I'm guessing the extra 3 are for competition for the sole backup spot, with the winner needing to be a good ST player.