A lot depends on your application and strength required. 4130 can be heat treated from medium to high strength, with tougness varying inversely with strength. The first thing you should do is find out what tensile strength (or temper) the base metal is. This will affect the amount of preheat that may be required for your application. Once you know that, general guidelines or experience can be followed to arrive at an adequate preheat temperature. Otherwise it may be trial and error. Or if you know the chemical analyis, Lincoln makes a preheat calculator that can provide good recomendations for preheat temperatures. Additionally, you need to know how strong or tough the joint needs to be. If it needs to be matching strength, you can find a matching tensile strenth electrode, assuming your PWHT is just a stress relief. If you are going to quench and temper the entire weldment, Carburize's post contains the proper information. If you are going to "normalize" the weldment as evelrod mentions, and you want matching properties, also use the 4130 electrode from ESAB. If you don't need matching properties, the 70xx or 80xx electrodes would be acceptable.
If the above information is not applicable to your situation, or you need more specifics, you need to post more specific information about the base metal and what properties the weld needs to have. The keys are knowing what you have to weld on and knowing what you need when you are done. (property wise)
