When Firefox 4 is released, it is not going to be as an upgrade, but rather as an installer. You do not need to remove the old version of Firefox. However, by default when Firefox 4 goes to install it will likely try to install in the same folder as your present Firefox install. You can however avoid this by selecting Custom on the setup type.
Then enter a new location (such as C:\Program Files\Mozilla Firefox 4\ or C:\Program Files (x86)\Mozilla Firefox 4\ for those running 64-Bit Windows Vista or Windows 7 on the following screen:
The downside to this is you will have two different Firefox icons on your desktop. But, should you find your not quite ready for Firefox 4, you still have Firefox 3.6 installed and can easily go back (doing so may break some of your extensions in Firefox 3.6 see Add-on(s) Disabled by Firefox for directions on fixing this). If you should choose to through the uninstall route, be sure to leave un-checked Remove my Firefox personal data and customizations.