Highland
Not sure if you would be able to, but I generally pack my 4 piece rods in with my clothes in my suitcase as a check-in. I put the rod tube in the middle of my clothes so there are clothes surrounding the rod (and gives some cushion.
However, if you are just taking the rod tube with you, I don't see the problem with taking it as a carry on. I think a lot of that depends on what else you are taking on as a carry on. Most airlines allow 1 personal bag and 1 other item (example: I would carry on my computer bag and my mandolin).
Check with the airline when you check in, but be prepared to put it in your suitcase if need be. Or else, you can check in you rod tube at the gate. As afish mentioned, have all your information on the rod tube.