Well i have had no problems and i cant log in on the main page or i would have gotten some credit for posting it. My site is located at http://windows.jaha.dk and that is where the second link points 2. I decided to post it because i love the program and since my site has to do with windows and programs i thought i might as well post it here as well.
Anyway about the 4 and 5 buttons what exactly do you mean?
myIE may use different key combos to perform those actions, so the mouse is programmed with different shortcuts than myIE uses. You should be able to program the mouse to do something like "alt + right-arrow" or "alt + left-arrow" if those keys work in myIE, as stuyb said.