You have to bear with me as i havent used 2003 in a long time. 2005 has the tabcontrol and i thought 2003 did. But i was using 3rd party tools too so they had them which is where my memory could be going wrong.
Anyway. It is basically a series of buttons that just look like a tab shape using an image. Then there are two ways you can respond depending on your needs. Either load all 4 forms in memory and when the relevant button is clicked use some clientside code (such as javascript) to display the relevant form/controls or use a normal server side postback and show the correct form depending on the button clicked.
You could always go so far as write your own usercontrol to do this for you (which is basically all the tools are anyway. Or depending on your needs... download VWD from microsoft which is the 2005 version and has all the new tools in it and is free (for a while)
