Here's an interesting link about braze welding, if anyone is interested.
It looks like a good way to get a strong joint in 4130, as long as it isn't exposed to high temps that would weaken the filler, and in this application that shouldn't be a problem.
If I were attempting this I'd have to make some test joints and test them in the shop press and/or have them fatigue tested, but that's because I don't have the previous experience that the OP has.
Here, when someone says bronze welding, they are talking about joining pieces where the base metal is bronze.
Normalized 4130 tubing can be torch welded, or tig welded, but preheating may be required. A professional welder who is an expert in these steels would be a must have IMHO. One stray arc could cause a future crack. E70S2 would be a good choice, as opposed to 4130 rod. Heat treated 4130 would likely require heat treating the entire frame after welding.
Anyway, super-cool project. I can't wait to see how it turns out.