As much as people bash the melodramatics of Wing Commander IV. It is still one of the greatest space flight sims.
Wing Commander IV branches from WC3, in that you're efforts leads to the "opposing" team to the Confederation and fight along side the Border Worlders against the tyranny of Earth. By far the most interesting addition to the WC universe is the inclusion of the Black Lance, basically the elite special forces of the Confederation that does the dirty work for Admiral Tolwyn, i.e. genocide (via bio weapons), attacking unarmed transports. Each faction's ships/planes are well varied and really adds to the scope of the game.