Hmm.. from Phenomenol's post.. Here's what I gathered.
The Yonkou are against the Marines.
The Yonkou are against the Shichibukai (sort of; Shanks and Mihawk seem pretty much neutral with one another)
The Shichibukai and the Marines are neutral with one another, and don't attack one another unnecessarily.
The Yonkou compete with one another for One Piece.
I don't know, but it seems to me that saying that all the powers are exactly equal could be oversimplifying matters. It looks to me that if the Yonkou didn't have some sort of advantage over the other 2, they could well easily be overrun.
The Grand Times article seem a little contradictory. First they said the Marines and the Shichibukai don't attack one another since they are both under the WG. Then they said that if the Shichibukai fell out of balance, the other 2, including the Marines, would attack them. So would the Marines attack them or not? Would the Marines attack the Shichibukai, the power they need to keep the Yonkou as well as possibly millions of other pirates in check? If I were the Marines, I wouldn't rush to attack the enemy of my enemy, of whom i am neutral with.
We also consider that the Yonkou don't exactly see neutral with one another. Seeing how dangerous it would be for the WG and the Marines for the Yonkou to form some sort of alliance, each of the 4 must hold tremendous power, and it wouldn't be impractical to conclude their level of power as compared to the rest of the world.
HOWEVER, here we have another point. The Yonkou aside, the Marines and the Shichibukai also have other concerns. Mentioned before, there are possibly millions of pirates out there, causing havoc all across the globe. The Marines have to deal with them. The Shichibukai also don't go against the Yonkou all day long; they have their own businesses to attend to. The Yonkou, on the other hand, have their businesses directly linked to the effect of the 3 Great Powers, that is to search for One Piece in the New World before the WG or the 7BK get to them. I wouldn't say their concerns are less, but they can focus their attention more on these aspects, and wouldn't have to worry about other things like other small pirates or revolutions.
So what exactly IS the balance of power? One word to estimate the situation would be: uncertain. Circumstances suggest they are roughly equal, but for the One Pieceans to treat them as so would be foolhardy. Each individual Yonkou must be able to hold their own; if they were to join forces under a single banner, then the Marines and the Shichibukai as we know it would be no more, but of course, we know that would never happened. Therefore we do have to understand that the Yonkou should have an advantage over the other 2 powers.