Wyatt Swaim of the "Tig Depot" has information about welding of 4130 steel on his website at:
http://www.tigdepot.com/faq.html
The Experimental Aircraft Association (EAA), based in Oshkosh, WI, also has a wealth of information about welding 4130 since its members have used tons of 4130 tubing, plate and bar stock for the fabrication of aircraft fuselages. The EAA's website is at:
http://www.eaa.org/