4140 is a low alloy steel with chromium and moly to increase hardness penetration and 0.40% carbon by weight. Industrially it, or something very similar, is used for a wide variety of things requiring relatively high strength will considerable toughness and ductility. It is also weldable, but a good procedure must be used. For axes and large knives where toughness is considered the primary attribute by a good margin, it might be a good choice. The upper limit of hardness is 56-57, but it is rarely, if ever, put into service that high. Axes, tomahawks, and entry tools come to mind as the best applications. For knives, edge holding will be adequate to poor I'd think, since even at the highest hardness it doesnt have any carbides to speak of for wear resistance. For large tough knives there are more suitable choices. Then again, I've never tried it, though I thought about it for a while. Its in the same family of steels as 5160, 9260, 6150, 1050, and others, but with higher toughness and lower edge holding, both directly related to its lower carbon content.
and ASP batons are made of 4140, too