It's extremely hybrid. It's a 4-3 alignment that plays more like a 3-4 and with a mix of 43 & 34 personnel. The major difference is they do not one-gap or two-gap straight across the board. The run stuffers two-gap and the pass rushers are set loose. The primary pass rushers are the Leo (weakside end), the SLB, and the 3' tech DT.
If you are familiar with the Seahawks from last year then you know that the Leo's last year were Chris Clemons and Michael Bennett (combined for 13.0 sacks), the 3' tech's were Tony McDaniel and Clinton McDonald (combined for 7.5 sacks), and the SLB's were Cliff Avril and Bruce Irvin (combined for 10.0 sacks).
Looks a lot like a 3-4 right? Clowney would most definitely be a Leo in their scheme as a pure DE. Mack would be the SLB who, while a major pass rushing threat, is still a 4-3 OLB and must be able to cover TE's and the like. IMO, it's just the next evolution of defense. The Seahawks have managed to merge the 4-3 and 3-4 defenses and find both sets of personnel to fit it. They have taken merging athleticism with skill sets to the next level.
Btw, I agree about Clowney. He is the top talent. My original point was not that they would take Mack over Clowney, but that they would take Mack if Clowney was gone and not be that upset about it.