Gotta love 1958. And people act like losing 9 seats is a big deal.
When you lose a majority in Senate (and most chances to regain it until 2020-2022) - it's a BIG deal...
Because gaining 4 seats is so implausible. 
In present political climate it will be difficult, Besides Johnson, Kirk and (IF he runs for President AND stays in that race) Rubio's seat i don't see really top-tier targets right now. And no, i don't list Ayotte's seat or McCain's (it's Arizona after all) among them so far. If a big Democratic wave materializes - then yes.
You forgot Toomey. Gaining three seats will be relatively easy if dems win the white house. Its the fourth that will be tough.
I didn't really forgot Toomey. He is rather smart - a solid conservative who doesn't irritate too many people. And Sestak has a lot of baggage too. In addition - western (Appalachian) Pennsylvania (except, may be, Pittsburg and Erie) took hard right turn recently, as most of other Appalachian regions did. So it will not bee so easy in Pennsylvania.
If Hillary is carrying PA solidly, its hard to see Toomey winning. That kind of ticket splitting rarely exists anymore.
