Wow! there are a lot of posts for this topic, and not being one to be left out, I will add my opinion here also. Although the question is vague as to what the application is, I will attempt to cover all the bases.

First, the use filler material that closely matches the base metal is not intirely true, as there are many factors that dictate what filler material to choose. FAA AC43.13-1A, section 5, 4-74 (9), states "Use welding rod and electrodes that are compatible with the materials to be welded. Welding rods and electrodes for various applications have special properties suitable for the application intended". Which means that one must carefully choose the correct filler material for the application, and the intended service of the part/structure.

According to AWS A5.18, the recommended filler for most applications is ER70-S2. The reason for this is that, 4130 is a fairly brittle alloy. Using a more ductile alloy filler material, will of course be fused and "alloyed" with the parent metal, which inturn will allow the joint to be more ductile and reduce the tendancy for tearing and brittleness at the joint.

Second, the reason that NASCAR and other sanctioning bodies disallow 4130 in favor of 1020 mild steel, is that 4130 doesn't posses the desired properties for energy absorbtion. It's just too darn tough. The low carbon mild steels absorb impact energy far better as they are more ductile. Ironically, early Piper airframes like the J3 Cub, PA-18, PA-20, etc., were all built from 1020 mild steel. Probably not for energy absorbtion in a crash scenario, but probably because of cost factors.

I have always used ER70-S2 for all of my GTAW joints in both aircraft and racecars, and I have always had either the airframe, or the chassis heat treated, (normalized), for stress relief. If you can't have the entire structure heat treated in an oven in a controlled enviroment, than pre-heat, and post heats should be employed, as well as maintaining correct inter-pass tempatures for critical joints. Most joints in aircraft, and in racing cars, are small and are of thin walled tubing which will dissapate the heat quickly, and spread the induced stresses away from the joint. Field stress relieving will help reduce this built up joint stress, but for complete structures, an oven is best. AISI/SAE 4130 alloy manufactured to MIL-T-6736, is a normalized and annealed seamless tubing possesing a tensil strength of 90,000 psi. This is the desired alloy for aircraft and most race car rules that require 4130 structures.

Finally, you must determine what your specific application is and what performance you expect from it, before you can choose the best procedure and filler material for it. Also, Tubing joints, and flat sheet cert tests are not comparable criteria, (MIL-SPEC 1595A), for standard bend tests. Again, each application is different.

I hope this didn't confuse you any further.