How much is a 4 day Disney Park Hopper ticket?
As of this writing (June 2012), a single day ticket for either
Disneyland or Disney's California Adventure (but not both; i.e. you
have to pick one and stick with it) is $81 for ages 3-9, $87 for 10
and over (there's no charge for children under 3, but you probably
don't want to be taking children under 3 to Disneyland if you can
help it).
There are tickets that allow you to go back and forth between
the parks, but they're about $40 more ($119/$125).
Occasionally there are special deals available. Sometimes these
require that you live in Southern California. If you don't, and
you've got travel expenses anyway, you should probably consider
multi-day passes... it's not like you're going to be able to do
everything at Disneyland in one day anyway.
Also, parking is $15 per day.