Top quality thread this, well done. Some excellent points made, too many to go into individually...
I do agree with Yorky about Hyppia being already the definite 4th CB, and I agree Carra is the man to be demoted in favour of the pacier, more attacking and more technically adept Agger - Skrtel combination.
I just can't see it happening before the mid-point of the season, however, even with the most ambitious timescale. there are just so many factors to consider in selecting the CB pairing:
Form (both individual performance and defensive-unit performance)
Opposition (1 vs. 2 strikers; pace; dead-ball ability; skill level; sheer awkwardness; numbers of attackers; attacking intent, etc...)
Competition (League vs. Cup tie vs. Champions league; keeping one eye on the forthcoming matches, etc.)
Need to bed in new players - With Riise gone, and Dossena & Degen to begin bedding in, and Skrtel to continue doing so, and everyone to get used to each other and the team needs, Carra (and Sami) will be needed to maintain organisation and discipline. Certainly if both Dossena and Degen play at the same time, Carra will play too initially.
Rotation of full backs, requiring stability at CB - each FB position will also be rotated. As the degree of familiarity with the new guys increases, the need to have Carra playing to compensate for their inexperience in the team will decrease.
Rotation of CB pairing , while needing to ensure experience / organisation / pace / stability etc. are maintained
Formation changes (3 CBs / 4-4-2 / 4-5-1, etc)
Injuries & suspensions.
I know I'm glossing over these points, but I'm sure you'll be able to interpret what I mean, and flesh them out for yourselves better than I can. If I went into them in depth, I'd be all day at it!
Also there's another point nobody has gone into much as yet. When playing the other "Big Three", home or away, we'll definitely play somewhat differently to playing any other teams - there'll be more outright defensive work to be done. Against everyone else, there's be varying degrees of attacking / defending to do, depending on whether the game is at Anfield or not, and who the opposition is. The likes of Stoke and co will look to defend all-out home or away, while Spurs and others (who?) will pose more of an attacking threat. I'll expect Rafa to rotate the pairings with this just one of the factors he consideres. He probably has a mainframe trying to work out the permutations!!!
All in all, I think carra and Agger - assuming Agger is fully fit and well - will probably be the starting pairing, until Rafa is happy that Agger is back to 100% fitness, and fully re-integrated into the team. If Agger is off the boil for any reason, then it'll be Skrtel instead.
Over time, I expect Carra will be phased gently out, but certainly not before the 2nd half of the season. Of course injuries or loss of form may completely alter that timescale. But if Carra is back to the best of his form, he may still be CB #1 right up to next season. Watch this space...