"Not good advice! The only benefit of using e.g. 4130 wire would be if after all welding is done the whole thing was going to be heat-treated to some elevated tensile condition. The MIG equivelant of E70S wire is appropriate for what you are attempting."

Good catch, mostly as regards using 4130 itself as a filler rod. I would consider any filler rod that is designed for high tensile applications with 4130 to be "4130 filler rod", though to say that gives rise to a classic top ten tig welding 4130 question.

As far as the heat treating is concerned, the same problems apply to the tubing itself. You are melting, slowly cooling, and not further heat treating the tubing at the weld also. The normal side of this argument that I have taken is to say you don't need materials in the filler that are stronger than strong enough. With enough fillet size in the joint you will be strong enough using the same stuff used for 1018, or something better, like er80s-d2 which i have to confess I have never seen for sale up here.

One reason people have given for using 4130 filler rod (not sure what it is actually made of, there are several versions of the stuff) is either A) it lays down better, B) it is stronger in a non-heat treated form, just as the tube has a higher tensile strength than a 1018, even if the tube is not fully hardened.

Reasons given for using the 1018 stuff or stainless are that it is more available, more easily laid down with gas welding, higher ductility.