Check with Bill at Doghouse Rocketry. He sells rail buttons and standoffs.
The standoffs will allow you run the launch rail through one of the tubes without rubbing...assuming your tubes are large enough.
In my case, a 1010 rail fits neatly inside a 3" diameter tube fin. A 4" tube fin should pose no problems at all.
BTW, Bill also sells wellnuts which will make installation a bit easier. Just drill a hole in the body tube, stick the wellnut in and it will swell up when you drive the screw home. Easy peasy.
If you forget to leave a bit of body tube extending past the tube fincan like I once did, you can drill a hole through the outside of the tube fin and then into the body tube, a wellnut, a standoff, a rail button, and Bob's yer uncle. You can fill in the hole in the tube fin if you like.
Note to self: Tube fin rockets don't like Mach.