A couple of misconceptions here, but hey - 3 years ago I didn't know any more.... 4x5 is "large format" not "medium" and as others pointed out, SLRs are not common in this size....
Originally Posted by Huram
The lenses for 4x5 cameras are interchangeable between makes - the only bit you may need to change is the lens panel that the lens mounts on if you use more than one camera ('tho there are adaptors between makes and some use the same size panel). To change an LF camera's lens you unclip the whole lens/lens panel assembly (usually, the shutter is part of the lens) and clip on the new one. Other bits such as film holders are likewise largely interchangeable between makes.
Have a scout around www.lfphoto.info for more info than you can shake several sticks at...
You also need to decide what you are going to do with those big negatives: 4x5 enlargers are large beasts and 4x5 is probably a bit small for most people to be happy with contact printing. The other option is scanning... (shhhhh - you didn't hear me say that).
Have a look on the auction site to get an idea of what is available in the real world and how much you might have to pay.