Yes. There is no reason not to have all four browsers installed, if you want all four.

Note that you can't uninstall Internet Explorer, at least not for most practical purposes, as it is integrated into Windows itself - which, of course, is exactly why it is so dangerous.

The best genral policy is to install your choice of Opera, Firefox,or Seamonkey (or all three if you want to), then delete the shortcuts to Internet Explorer so that you don't start it by accident.