The installation of MetaTrader 4 relies on the installation of Internet Explorer 6 for many connectivity libraries and functions. The installer files for IE6 were available exclusively through the Microsoft Download center, however Internet Explorer 6 has recently been removed from Microsoft's Download center. New installation of Internet Explorer 6 are no longer possible. Even if you already have the installer file for IE6 in Crossover's installer cache (which would be the case if you've already installed MetaTrader 4), that file will "phone home" during the installation process, and the files that it needs to pull down from the Microsoft Download center are gone.
This means that, for the time being, new installs of MetaTrader 4 are impossible.
If you have an existing install of MetaTrader 4, please use Crossover's 'Manage Bottles>Advanced>Make Archive' function to create an archive of your existing install for safe keeping in case you should need to delete your current bottle and re-install. A restored archive should work, but a re-install will not.
Our development team is working on a solution for this problem.