Well, this is all theoretical, and has almost nothing but my form of logic for it. I seem to think we can see how powerful a group or organization is by what they control.. One can perceive that the 'less' you control, the 'less' you have to do, so strength would be somewhat irrelevant if you had your hands overfilled. However the 'more' you control, the more influence you have. From what we have seen:
Marine Headquarters - Marines control the Whole World, however, the main focus of the headquarters is 'The Grand Line', and the Four Blues are almost void due to ignoring them or having their hands full, so we'll say the 'Marines control the whole world, but the [Headquarters] deals with the Grand Line.
Shichibukai - The Shichibukai are Seven Warlords of the Grand Line, speaks for itself. Their influence is needed on the Grand Line to keep order.
The Four Emperors - They are only inhabitants of the latter half of the Grand Line, or The New World.
Going back, I said the 'more' you control, the more influence you have. Thus we knew about the Marines since the beginning of the Manga, but we didn't learn of the Emperors until just before the New World (435ish). However, the Four Emperors obviously have the most power of the three, atleast seperately. The Gorousei are doing all in their power to prevent Red-Hair Shanks and Whitebeard Newgate from meeting because it'll "Disrupt the balance of power". Meaning they are only on par with the Four Emperors (IN THE NEW WORLD) while they are seperate. Its the price they pay for wanting to control the whole world.
So.. Conclusion? Quite simple.. On a power scale, the Shichibukai are not last, however, their numbers conclude that they are inferior to the Marines, however, because of this, their influence on the Grand Line is very helpful to the Government, who wants to control everything. The reason the Four Emperors are leading is because they work alone to fulfill their goals, control is not apart of what they want to do. If they happened to work together (which is incredibly unlikely) the World Government would be in a hell of a pickle.. To be fair though, Wheres the Rebel Army leader Dragon in this equation, his influence is pretty big too.. I just wanna watch the Manga/Anime and see the story progress, Kuma is cool, but seems to be overrated.