I bought a few planes and a honing guide at some garage sales (total
expenditure $12), and have been having fun playing with them; but have some
questions.

1) What is a short plane good for; why would you want to use a #4 rather
than a #5? Wouldn't that be like using a short jointer when a long one is
available? I suppose it is lighter, but what else.
2) One of the Stanley planes came with a Dunlap iron. Is that likely to be
better or worse than the Stanley irons?
3) An article in a "Fine Woodworking" book said that a hair's width off
perfectly flat on a #5 might be okay for rough work, but for precision work
it has to be better than that. Is this to be taken seriously?! I find it
hard to believe that a hair's width could even be measured over 14" (by the
home woodworker anyways), let alone make a difference.