I have not seen or cast the Hellbender, but I think they advertise it as being their big fish rod, so it is probably pretty stiff. Size 4.5 level line is plenty heavy so it should work, though. Switching to a furled line will not improve your form. It will allow you to cast with bad form so you might never improve, though.

I have a good friend who is a tenkara angler and also a western fly fishing casting instructor. Although most tenkara anglers use much more wrist, it you have a good western cast you don't have to change much at all (at least if you follow the Joan Wulff school of casting rather than the Lefty Kreh school of casting!). The one thing you absolutely must change, though, is the followthough. Do not, Do Not dip your rod tip at the end of the forward cast. Keep it high - no lower than 45 degrees. The basic physics of casting a line, whether western or tenkara, are identical. You must smoothly accelerate on the back cast and stop the back cast abruptly with the rod just past vertical. Pause until the line straightens behind you. You should be able to feel it tug on the rod when it does, but watch the back cast in case you haven't yet noticed the tug. As soon as the line straightens, start your forward cast. You must accelerate throughout the forward cast and you absolutely must stop the forward cast abruptly, with the rod no lower than 45 degrees. Almost everyone who has tenkara casting problems is doing two things wrong. 1) the don't stop abruptly enough. 2) they don't stop the forward cast with the rod high enough.

The cast should not be just wrist. You should move your upper arm, forearm and wrist. Review Joan Wulff's on-line videos of a fly cast. Her comments about the wrist snap that ends a forward cast - like using a screen door - push with your thumb (most tenkara anglers grip the rod with the index finger on top rather than the thumb, so push with your index finger) and pull with your ring finger and pinky) are THE key to a tenkara cast. That will give the rod the loading it needs so the rod tip whips forward when you stop your cast ABRUPTLY. I suggest Joan Wulff's videos rather than video's of Sakakibara because she describes what she is doing. He doesn't speak English so he can't tell you step by step what he does. But remember, do not dip the rod at the end of the forward cast. That is the one big difference.

I would slightly disagree with David and suggest starting with a line (not counting tippet) that is no longer than the rod. Once you get it to roll over properly, then go to a longer line.