ER630 is the common choice for 17-4 PH. Austenitics (specifically 308) have been used but will not have the tensile strength and will not have the PH response to heat treatment.
ASM Handbook Volume 6 is one of the best. Its pricey, but the volume of information contined in it is worth it, and indispenable to the welding engineer.
I would not use the ER630 to weld 17-4PH to 304L since the ER630 is not as user friendly as the regular austenitic stainless steels, and since the strenght of the 304L is adequate, there should not be any need to match the strength of the 17-4PH. I haven't done any math to figure out where the weld composition would lie on the WRC or Shaffler diagram, but it might not be pretty there either, and it depends on what process and joint you use. AWS D1.6-2007 has a nice table in the back recommending filler metals for all kinds of dissimilar stainless steel joining. My first guess would be to use 309L, but you should confirm that on the WRC diagram once you know your dilution rate, and test it in accordance with the proper welding code.