Most would recommend you set your document size to the trim size of one post card. When you're done with the design, you can go four-up on a sheet by making a new document large enough to fit four and place the original file as a graphic. You don't need to resave the original file as a graphic first—you can just select it the same way you would select a tiff or PDF, but you may export the original as a PDF if you prefer. The advantage of placing the original is that you can't make the mistake of updating the original and forget to update the PDF. And if you aren't printing this yourself, you should ask the print shop what they want (which will probably be a 1-up PDF that they can impose, since they know more about their requirements than you would).
If you get a file that you would like to reuse (maybe it has the size, colors, styles, etc. that you will need), you can save it as a template, rather than as a document. When you open the template, it will open as an untitled copy, and you won't run the risk of getting a bug creeping into the file that sometimes comes with constantly revising of older files and resaving them.
Also, since you're a newbie, you might want to check out this book by Sandee Cohen, which has been recommended by many of the forum users.