Raid 5 will probably be your best fit.
Here is a pretty basic site that will answer some of your questions.
RAID - Wikipedia, the free encyclopedia
In a home solution a nas might be the simplest solution. The nas, since it is connected to the network, will allow multiple machines to connect, versus a raid enclosure that will generally require the connected computer to be running for access to any other device. Unless you don't foresee a need to share to multiple machines, then the enclosure is fine.
I run five drives in my desktop with a secondary two drive nas, so four drive isn't ridiculous at all. Buy five drives, put four in the enclosure, one on the shelf. One drive dies, replace it from the one on the shelf and move on. The raid array will just rebuild itself.