Bear in mind 4130 is better suited to mass production, so is more likely to end up on cheaper machines. At the other end of the scale it can be triple butted, drawn and seamless, and on some Tange and Columbus tubesets can be as thin as 0.4mm in the centre of the cross bar. Similarly, 531 is available in heavy plain gauge gas pipe flavours as well as fancy double butted racing tubesets. ts how the engineers deploy it that matters. My Claude is plain gauge 531 and is comfortable but relatively uninspiring, but by double butted CroMo Alpinestars is flighty and r, only because that's how the engineers made them.
In the case of the OP I suspect the frame is of moderate quality, perhaps double butted main tubes. The componentry isn't indicative of a top flight frame (though you never know) or a low end Argos special either. Best guess is somewhere a little south of middling, worth taking the time to strip, clean and lube to see it give another 25 years service.
