The most important thing to think about when you're looking at contemporary steel disc road bikes is not the material, because a lot of them are 4130 or alloys like it. It is how judiciously the material is used in terms of weight. Many mainstream steel disc road bikes go way beyond reasonable in wall thickness and tube diameter spec. For example, it's easy to buy one that has a 38mm downtube with .9/.6/.9mm wall thickness. That is a heavy tube and it used to be the domain of freeride hardtails, but buyers are acclimated to the chunky look now and so many DTC steel bikes have it all over, and wind up ludicrously heavy. The most important to understand is that when someone says "4130 bikes are heavy," "531 bikes are light," the sole piece of information that person is telling you is they don't understand how steel frames work. Weight and performance are factors of base material and tube spec choices.
Surly tends to do a more-reasonable-than-most job with this, and they dial the diameters and wall thicknesses to individual size bikes more than most brands.
In terms of the material itself, 4130 is a standardized designation. In other words it refers to specific proportions (or range of acceptable proportions) of iron, carbon, molybdenum, and chromium. It has strength and toughness properties far beyond high-tensile steel. It also has somewhat less ductility than some other steel types used on bikes, so there are areas like dropouts where other materials might be chosen instead. There are many bicycle tubeset products that are much like 4130 and perform only incrementally different in any direction, but add manganese and some other elements. Reynolds 531 and Columbus SL are examples of this. All these materials do more or less the same thing functionally for the same tube specs. From there you move to much more expensive "super" steels that are much stronger per weight, and therefore can make frames that are lighter for the same strength or stronger for the same weight. In practice, 4130 and other classic materials often perform better in terms of actual toughness and longevity than such frames, but are heavier.
