Re: No wrist = fly hitting rod tip... ???
Here are some things to think about.
1. 3/0 and 4/0 flies are big flies and if they are weighted or have eyes or cone heads, then that makes it even worse. I think you are losing your back cast line speed and the big flies are falling below the rod tip. Have you tried to use a haul to help line speed?
2. As suggested by Jackster you can alter your back and fore cast planes slightly so they don't collide. I don't think this is you problem though you should try it.
3. Shorten the amount of line you have out. Too much line in the air with big heavy flies can cause your problem. But then it is the same problem as not having enough line speed.
4. Since this is happening on the fore stroke you may be applying too much power to your fore stroke. You can increase line speed with out using a lot of power in your rod hand.
5. Try shorting your casting stroke in conjunction with a double haul.
So I think you need to experiment a bit with some of the suggestions in this thread. I would try a lighter fly with less line in the air and a shorter casting stroke. Try this from shore and not form the float tube. You need to pin down what your problem is before you jump in the tube.