Ausatco: Yes, funny that! The same for me. I 'appeared' to have ie4 on the system, but when I 'reverted to previous' it came up with a version of 5.0 which was not fully functional (and not showing on 'remove programmes' panel!) and would not allow re-install of 4 or 5.
W98 renistalled, no change. Finally took my courage in both hands and deleted the internet explorer keys in the registry (not for the faint-hearted!) and so far have now got 5.0 on and apparently working ok (and showing as in installed, which is progress!) and am about to try and reinstall 5.5.
Bill Gates I love you - not!