I have read that 4130 is generic Reynolds 520. This is a step down from 531. Perhaps someone can confirm.
I have a recent Reynolds 520 bike (Jamis Satellite), a Hi-Ten "1020" Pansonic, and two early '80's Trek frames, one is 520, the other 531.
Between the Jamis and the Panasonic, I like the Panasonic best! Another example of fit and geometry being more important than finer distinctions between frame material.
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The bicycle, the bicycle surely, should always be the vehicle of novelists and poets. Christopher Morley