Where is the engineer during these discussions?
You have provided scant information, yet you are asking us to provide a recommendation without telling us the application, the code or much of anything else.
Al
M_inspector
Those two materials can be welded together, but there are some items that need to be discussed with the welding engineer.
1. X56 is a P1 Group 2 material and has UTS of 71 KSI.
2. 4130 is considered an unclassified material under ASME (If it's A519-4130) (Unknown)
3. Need to know the code of construction. (Unknown)
4. Typically 4130 (in my world anyway) is to be PWHT where X56 does not, unless its 1" or over. Again depending on the COC. For example. Under ASME your given exemptions. (Unknown)
5. ER80S-D2 is a 80 KSI material but that could change a little during PWHT, if required. So, material match to filler metal looks ok.
As Al has already indicated. There are to many unknowns. You need to get a welding engineer involved.
My thoughts anyway.