Interesting.

I would need more information to make any meaningful comments. However, even if the stress relief temperature is in terms of Celsius, it seems a little on the low side. I usually like to see some preheat with 4130, even the thinner material, to mitigate the possibility of cracking upon cooling.

The filler metal doesn't match the strength of the base metal. However, that isn't always a bad thing. It typically provides better ductility at the expense of strength and hardness.

Stress relief is typically 15 minutes per 1/4 inch of thickness or 1 hour per 25 mm of thickness.

However, my opinion is of little consequence if the customer has specified the post weld heat treatment on the drawings. The drawing is the bible until it is changed. 

I'm just wrapping up a repair job on a forging made using AISI 4140. I undermatched the filler metal by using 4130, but there is some strain hardening expected on the 10 inch by 10 inch cross section. We performed three in-process stress relief operations and a final normalizing operation. No cracking problems were observed. Our preheat was 500 degrees min and the maximum interpass temperature was 700 degrees F.

The thinner material you are working with does not require the high preheat needed for the thick section. As for cooling the welded part to room temperature before stress relief, I wouldn't recommend allowing to cool before S.R.

Good luck.

Al