It used to be common practice to label a mostly hi-ten(carbon steel) fram as cromoly if they had some cromo in it. Such as a seat tube or a downtube. It was sneaky. So far as I know that doesnt happen anymore fortunately.
I personally love 4130 cromo. It is just about the toughest frame material out there, so long as the rust is prevented(which isnt hard).
The difference is in what they CAN do with the two. Carbon steel is very soft as compared to cromoly or aluminum. The typical huffy frame(which is the prime carbon steel) the rear triangle can be bent open or closed with one hand(I had to junk a few for science in childhood). Carbon steel rusts relatively easily. Due to all these traits, they have to make the tubes thick. AKA heavy.
4130 Chrome-Molybdenum steel is second only to titanium in overal strength but is far cheaper. It has a much higher spring stregth as well, which makes a frame rugged. That gives the 'lively' feel people including myself rave about. Due to its strength, it can be thinned out greatly and still be useful. On the better frames, the "butted" frames, they can taper the tubing to some amazingly thin diameters and still not worry. I was totally amazed when I welded a part of mine, the stuff was paper! But it made it through a 20 mph hit on a sharp rock, then the remaining 7 miles of drops and shutes to get home without bending anymore.
Some of the new generation of Cro-mo frames are getting almost aluminum wieghts. One I saw was down to 4 lbs for a large sized mountain frame. You will not likely find any suspended designs on the shelves with ferrous tubes anymore, but the dirt jump and hardtail freeride guys chose 4130 for a reason.