I would say that Roach and Mack are the LBs to own. Mack is a versatile player with great pass-rushing skills, instinctive abilities that makes him effective against the run, and "underrated" coverage abilities that are talked about so often that I don't think we can call them underrated any longer, so I would expect him to get a lot of snaps. I'm pretty sure Tarver will use a few different looks so I think it is a bit too simplified to say that Mack will play WLB or that Mack will play SLB. In some looks he might line up as RDE, some times at WLB, and some times at SLB. He might even move inside on some 3-4 packages. But with that being said I would expect their base 4-3 to include Mack at WLB and Moore at SLB.
Sio Moore played SLB last year where he played 57% of the snaps and his primary use was as a pass rusher. Burnett played WLB and played 91% of the snaps, but his skill-set is best suited for pass coverage as he struggles against the run. He can be disruptive in the backfield but he misses too many tackles. Roach is a bit of the opposite as he is not great in pass coverage but he excels against the run. And then they also have Miles Burris who adds depth as a physical guy against the run. In obvious pass situations I could see them play nickel with Mack at RDE, Moore as a pass rusher, and Burnett or Roach in pass coverage. Or they can have a 3-4 (they drafted a NT in the 4th) with Mack and Moore/Woodley at OLB and Burnett and Roach inside. On running downs you could see Mack, Roach and any one of Burnett/Burris/Moore. Roach played every snap last year. If he does so again this year, both Moore and Burnett would likely suffer. Alternatively you could see everyone lose snaps due to the addition of Mack, but for sure Burnett is the one who is most likely to go down drastically from his 91% last season. I'm pretty certain that he will become a situational role player this coming season. I would also say that Moore needs to show himself to be much more effective against the run if he is to get meaningful playing time for fantasy purposes. His tackling efficiency and run stop percentage was near the bottom for 4-3 outside linebackers last season. He was very productive in college, but I guess the question is if he has the range and closing speed to do the same in the NFL.