I believe it should include the starting fuel tank. so 4 + starting fuel tank.
We weren't sure whether the "Have 5 different Improvements" condition is supposed to include the Starting Fuel Tank or not. From a strict reading of the rules, it looks like it should.
During the game, without stopping to check the rules, we mutually agreed to not include the Starting Fuel Tank, and that we would need to buy 5 *additional* improvements to trigger a win.
Not a huge deal, but should probably be clarified to avoid one person thinking they won with 4+1 and their opponent upset because they thought 5+1 was necessary.