Lawrence,
You make some very good points. Please allow me to voice my opinion. The carbon content of 4130 is 0.28-0.33%. For thicknesses up to 0.5" the recommended "minimum" preheat and interpass temperature is 300F. For HTLA steels (4130), 4 things determine the preheat and interpass temperatures: 1) the carbon content and alloy content, 2) condition of any heat treatment, 3) section thickness or amount of joint restraint, and 4) available hydrogen during welding.
Because of their high hardenability, it is necessary to preheat these steels properly for welding in order to avoid hydrogen cracking and to obtain a sound weld. Controlled preheat and interpass temperatures are necessary to prevent excessive hardness in the weld metal and HAZ. Actually, 4130 is one of the HTLA's with the lowest carbon content. Only 4027 and 4320 have lower carbon contents than the 4130. A few others have the same, but not lower, so preheat and interpass temperature is very important, probably moreso than PWHT. In the 4130, the most deciding factor for preheat and interpass temperature is the thickness, more than the carbon content.
Chuck