Blackie and L6,
In answer to your questions, I'll offer my personal thoughts on why an "Anvil-Shaped Anvil" is slightly more attractive to me than a solid chunk of steel.
First off, Blackboogers, I'm not certain of the specific properties of 4140, but it seems to me that anything that can be ground flat, and then face-hardened, should work as an anvil. This is to say, it works in the same way that anything reasonably heavy that you can tie a rope around, will work as a boat anchor. You can use it, but it may not be the best design for the job.
Bridging to L6Steel's question, the anvil has developed that shape because it incorporates a lot of surfaces and functionality that a blacksmith or bladesmith might need while forging a blade. The face is wide, but not so wide that it just sucks all of the heat out of the work before you can get in a bit of work. The horn allows you to work on curves and rounds, without having to worry about moving to another anvil. The hardy and pritchel holes serve both to allow for punching and the use of attachments--which, again, add to the functionality of the anvil without necessitating having a half-dozen clunky hunks of steel around your forge (the same logic led to the evolution of the swageblock, I would guess).
Certainly, I've seen other types of anvils, and I'm told by such experts as Don Fogg (via his website, of course--I should be SO LUCKY as to be "tight" with Mr. Fogg) that they work as well or better for their purposes as anything Peter Wright or MouseHole ever dreamt of. Me, I do other blacksmithing than just bladework, so I think it would be useful to have all of the various shapes right there. If you have access to big ole' chunk of something with a flat surface, and you think it'll work for you, then by all means, forge ahead (pun intended). I have a railroad track, which has served me well enough thusfar, but I find that it's a pain that it's not wider, it has no good curved horn, and lacks a hole to do piercing work on. Thus, my desire for something else.
My main concern was to see if anyone had experienced anything untoward, such as hitting a piece, and cracking the anvil face or some similar misadventure. I think I may swing by HF tomorrow, and see what they have to offer. I need to pick up a table saw, anyway...
