Something must be seriously wrong with your computer.
You say:
" I had somehow lost my last version of lightroom from my computer. I can only open lightroom 3 prior to my upgrades. . . all my recent catalogs and pictures in lightroom disappeared."
This doesn't happen just on its own. Either you did something or somebody else did something, or you have malware on your computer.
But have you searched for your photos in Mac Finder? Searching and looking with Mac Finder is the only way to tell what is on the hard drive and what is not.
And you say:
" It is saying "your disk "My book" which contains your Lightroom catalog is critically low on space."
If your disk is full and you don't want to delete anything, then it is time to
a) either get an external hard drive and put files on the external to free up space on the internal drive;
b) or get a new computer.
The hard drive that contains the Lr catalog needs free space so that Lr can work with the catalog. "catalog" doesn't mean photos, it means the database that Lr works with and where everything that you do in Lr is saved.
It seems you have your Lr catalog on your external drive. If this is the case, you can move the catalog (and again that doesn't mean photos) on an internal drive where it probably will work better. The catalog, i.e. the file with the extension < .lrcat> does not have to be on the same drive where your photos are.
To move this file: Find it in Mac Finder and move it to an internal drive where you have more space. Then double-click the file to open Lr.
And you say:
"This is why I thought I might as well buy Lightroom 4."
With the current state your computer is in, Lr4 won't help you much.
But for all that it's worth, no, you don't have to uninstall Lr3 to install Lr4.
You say: "I am very confused and wish I could talk to a person."
There is a Chat line, see here: http://www.adobe.com/support/chat/ivrchat.html