I am not sure if Air Berlin are really interested in FlyBe. If so, I don't think it has much to do with the Q400 but more with the need to get access to the strategically important UK market. There have been quite a few Q400 short-term delivery orders recently, so if one needs some, one can get some.
Air Berlin certainly needs a critical mass in able to survive as a major independent LCC in the long term. They have been able to grow in places like Switzerland and Spain and now begin to penetrate Italy, have joined forces with FlyNiki to capture the Austrian market, but as a true European LCC they certainly need a foothold in the UK market - which certainly is much more difficult to conquer than all other European markets because if ots well-developed LCC scene.
Air Berlin's STN mini-hub is a nice try, but very much a niche operation. A big-scale assault on the UK-market certainly would require some sort of link-up with an established carrier. With the bmibaby/Lufthansa connection, bmibaby as, imho, the most likely target is out of reach. Jet2 would make some sense, but I guess that with the whole ChannEx stuff on top of it, difficult to get hold of. That pretty much leaves FlyBe as the only possible take-over target (or maybe just a co-operation partner?).
In addition to that aspect, the FlyBe business model certainly has some appeal to the German/Austrian market. There are a lot of smaller airports that cannot sustain a 150seater LCC operation but could nicely do with some based Q400. Just think of all the smaller Austrian airports such as LNZ, SZG, GRZ, KLU and INN or airports in Germany such as FKB, NUE, DRS, BRE, PAD. However, FlyBe will find it difficult to establish themselves on those markets as a relatively unknown UK airline.