I believe you have to read a little deeper than you are. Table 4.8 uses the combinations of the groupings (of the steels) listed in Table 3.1 and/or the base metals listed in Table 4.9. Depending on the combination of groups, you may or may not have to do additional qualifications.
"if i qualify a test plate of 2 A514's"
Simply qualifying two plates of ASTM A514 doesn't automatically qualify you to weld any of the base metals listed in Table 3.1. Instead, it says that you can only weld base metals having the same specification (ASTM A514) and the same grade (or type), and the same minimum yield strength as listed in the PQR.
As for the backing material, usually the backing can be the same as either of the two "dissimilar" base metals being welded.
If you are qualifying the procedure, you can do what you wish provided the final results complies with the minimum requirements of the code. If you are using an undermatch filler metal, it stands to reason that the weld may not have a tensile strength or yield strength that is the same as the "stronger" base metal. This can work to your benefit in highly restrained joints where it is desirable to have a weld with lower strength, but better ductility to accommodate the contraction that will take place as the weld and surrounding base metals cools to room temperature. I've used this technique for maintenace welding of large casting. However, I usually limit the reduction in tensile strength of the filler metal to 10,000 psi so as not to produce a weld that is "very" under strength compared to the base metal being welded. The published tensile strengths of the filler metal (per the AWS/ASME filler metal specification) are typically lower than the actual tensile strength produced by the weld.
Good luck - Al