I currently have an aircraft welding business that does repairs,
modifications, and fabrication of 4130 steel, 321 stainless, and all
weldable aluminum alloys. To answer the OP's question, yes, you can
weld 4130 with an OA torch. As to the rest of the discussion, yes,
it's alot like politics and religion...lots of opinions, all dearly
held. So, here is mine, based on many years of daily application.
All fixed wing OEM's that I know about MIG weld airframes. It's
faster and cheaper. Also none do any pre or post weld heat treatment,
as far as I know, and neither do I. Same goes for major kitplane
OEM's. I have been doing more MIG welding as time goes by, for the
same reasons. After-market airframes are mostly TIG welded, I think
because they are more esthetically pleasing to a potential buyer and
because MIG welding thin-wall tubing to aircraft standards is much
more challenging and therefore hard to find and keep competent
weldors. TIG is the same as OA in that you have a heat source in one
hand and a filler in the other. With TIG you have much better heat
control and now with the availability of TIG welders is very popular.
I would recommend TIG or OA for the homebuilder using ER70S-2 or ER80S-
D2 filler. I would not recommend MIG for the homebuilder, even tho you
can get a MIG welder for very little money. They are a different
process and much more skill and experience with thin-wall tubing is
required to do consistent airworthy welds. And triple that experience
to get as nice looking welds as TIG.
Good luck and ... practice, practice, practice in combination with
competent instruction.