This is a pretty funny discussion... you do know that 4130 is simply a designation number for steel right? All 4130 means is that the steel contains other elements in a defined percentage range.
As far as American vs. anything else goes, that is a goofy way of deciding on a bike frame. Name some tubing manufacturers that are well known in the cycling world, Reynolds (England), Columbus (Italy), Tange (Japan), Sanko (Japan), True Temper (US)....they all make all sorts of tubing that meets the criteria to be called 4130.
Sanko tubes are considered desirable as they are "drawn" tubes, meaning the tube was formed through a forging process rather than formed by rolling a sheet and welding it along the seam. Does that mean it dents easier, I doubt it. What would make a frame dent easier is how thin the tube wall is at the spot that gets hit. A butted tube is made lighter by having thinner walls in the middle and thicker at the ends where it is welded. So if you want a heavy frame that resists denting don't get butted tubes.
What is going to make more difference is the frame design and the welding, a poorly designed frame with bad welds and the best tubing available is still a piece of junk.