Think about how many mics/line ins you want to record at one time. With a 4 piece band how many vocal mics, how many instruments, and how much do you want to mic the drums?
Then you'll need to get a soundcard to connect to the computer. Any soundcard with 8 ins/out is typically going to be a very high quality soundcard. For example, the Delta 1010 has 8 analog inputs so you can go live into 8 tracks at a time to your computer (Aardvark Q10 and Echo and MOTU and others also make similar cards). Once you've got your tracks on the computer you can add as many as you want in addition to the existing tracks. If you decide you need more than 8 tracks recorded at once, you'll need to buy 2 or more cards and chain them together.
Another possibility is to get a mixing board (I got a Mackie 1642VLZ Pro) and mic / line in your instruments and vox and then send a stereo mix to the soundcard onto 2 tracks. If you do this you can get a less expensive soundcard to connect to your computer.
If you've got a good fast computer w/ lots of RAM then you can add many many tracks for any given song, but you still will be limited to what your soundcard can input at one time.
One common way to record a band is to get everyone together to play, but only send the drums and bass and a scratch track through the soundcard to the computer. Then you can add your other tracks such as vox and guitar etc. one at a time and build from the ground up.
There are books in your bookstore that can help you learn, for example there's a new "Home Recording for Musicians for Dummies" that I found useful. Also keep reading this board and other online resources and you'll learn a lot about how people record and the many many options about gear to buy.