As with most answers to science questions, the real answer is 'it depends.'
Earthworms are annelids, or segmented worms. Nightcrawlers, the big wiggly worms often used as fishing bait, have about 150 segments, while manure and red worms (red worms are also fairly common) have 95 segments. Different segments perform different functions, just like our body parts. We don't expect an arm to do what a toe can do, and the same thing is true for the segments of a worm's body.
Here's a great site with tons of worm info, and lots of drawings of
http://members.aol.com/camusurca/earthw ... rtworm.htm
Regarding your real question: Earthworms run into this problem all the
time, since birds like to eat worms. When a bird tries to yank a worm out
of its burrow, the worm uses bristles on its skin to hang tight to the wall
of the burrow. If the bird pulls hard enough, it can yank off part of the
worm's body. If the worm is broken off at the first seven or eight rings,
it can grow new segments and will survive. If the worm is pulled in half,
only the head end grows back.
So, if you cut a worm into four pieces, you most likely won't get four
happy worms. If you cut a worm into two pieces, you may get two worms, if you don't cut too far down the worm (only 7 or 8 segments down). If you cut the worm in half, you'll get one live worm after the head end grows back, but the stress of being chopped in two may kill both ends. I'm guessing that being split in half is hard on the poor thing.
This information came from the following worm site, which has all kinds of
details about earthworm anatomy and natural history: