Depending on who you ask, the desired filler for 4130 is ER70S-2, ER80S-D2 or even 312 (stainless). Most seem to lean towards the ER70 because of it's slightly lower strength and weld qualities. It's not quite as finicky as the higher strength material. The ER80 when mixed with the base metal, approaches the hardness of 4130. The ER70 weld doesn't get quite as hard. The result is the weld has a bit of "give" which can help prevent cracking or breaking in an impact situation without sacrificing strength in a properly designed assembly. I use ER70S-2.
TIG welds on properly cleaned, prepped, and welded joints will be pretty. If not, you need to fix what's wrong.
I used to think the same thing that the joint need to be more tolerant to stresses to prevent cracking but after much research and debate I have learned that its not always so. There are a lot of Myths on 4130, some aircraft guys still preheat and post heat to a dull red and let air cool to normalize. This prevents cracking alongside the heat affected zone.
I have actually talked to the engineer who designed the welding procedures for ASTM, the reason they are what he wrote them was because it was easy to weld and they made it easy for testing to destruction purposes. For ease of welding I have used RG45 to practice with but some organizations only allow 4130 filler rod while others accept er70s2/er70s6, etc.
Also oxy/acetylene makes some of the most beautiful welds on 4130 and is able to fill a much larger gap than a Tig can in a single pass. Most of what I weld is .035 wall with some .020 and .049/.058 so its not as thick as a chassis shop would use. Mig wire also works as a good filler for the thin wall tubing. The Tinman has a lot of good videos on welding and I still prefer a torch over a Tig because I am more comfortable with it. Tim
