As others have indicated, getting a 600mm f4 into a backpack and onto a small plane is do-able if your are resourceful and have big vest pockets for other stuff. IF it's just a matter of limiting the carry-on while also checking a bag, you can put the hood into the checked bag, along with your tripod, gimbal, and flash gear (if any).
The issue is, how much other high-value optical stuff do you have to deal with and what to do with it? I was tempted recently to sign on to a trip to Belize that included a return to Belize City at trip's end on a small Tropic Air plane that has no bin space and a 15 pound weight limit for lap-held carry-ons. If all one had was a camera body, a 600mm f4, and say, two TC's, you could put these into a GuraGear or Mindshift backpack and also your pockets and probably get away with it. But in my own case, I would also have binoculars, at least one more lens (a 200-500mm, say), a point and shoot for scenics, lithium batteries, a few other odds and ends. Getting the pack under 20 pounds would be a real challenge and my pockets would be stuffed. I am the nervous type; this is not my preferred way to do things. This is why for international trips I now use Olympus M43 gear; it is smaller and lighter and these kinds of situations are do-able
Don't even think about using a rolling bag for small plane flights, as even the major airlines will very likely force you to gate check them on regional planes like some Bombadiers. Last May I had to gate check my ThinkTank International at the gate in Dallas for a short flight to Houston. Ugh. Gate agent was completely intransigent. Gate checking is not as onerous as regular, in the main hold checking, and nothing bad happened. But I sure didn't like it.
More and more people are going the Pelican-case route and checking the bag. This prospect makes me really nervous. But I guess it's an option.