You have the solid bar and you're trying to confirm that it is in fact the piece that the mill cert applies to?
Or you have the solid bar and you're trying to confirm that it is in fact 4130?
Or you're trying to confirm that 4130 can be used for manufacturing your product?
For the first two questions, there should be some kind of marking (stenciling, stamping) or tagging on the material itself. Some, maybe all, of the steel standards will specify how a piece is to be marked. Your QC program should address how markings are transferred.
If it's unmarked, some standards may have provision for using material of unknown source- check the standard and see. For a small amount of material, it likely wouldn't be worth bothering with.