If you want to boost...

You'll need:
A (old)
B (old) >> C (new) >> D (new)

And then A can exceed the group by however many levels, and BC and CD are valid RAF links so BCD will all get the 3x experience.

The other option is:
A -- C
B -- D

In which case, all four accounts get RAF to 60th, but you cannot boost at all, as C is dependent on A being similar level, as is D to B.

If you want to play four toons, and you're sure of your team, and fairly sure you don't want other teams, this might be the way to go.

I'd probably still go with the A // B - C - D approach.
Just make sure whichever of your two existing accounts has a good booster is on A; in a perfect world, one of your two accounts would have the booster and be close to full, while the other would have few toons on it... that way you could boost with A, already have toons there to join the teams you boost, and B has slots open for new toons.

Also consider going with five accounts, instead of four...
That gets you:
A (old) >> C >> D
B (old) >> E
Which means you can boost with A, and have RAF on BCDE.
If you were to boost four toons on BCDE, you would have:
A x0 // Plus x2 from C (granted levels) // Plus x1 (Cascade granted levels)
B x4 // Plus x2 from E (granted levels)
C x4 // Plus x2 from D (granted levels)
D x4
E x4
Also a party of five gets you a full group for instances.
In which case, I'd go Pally and 4x Shaman if you had a PvE focus.
Or a Druid and 4x Shaman if you had a PvP focus... Druid can be both a Bear for PvE and a Boomkin for PvP, which brings the 13% spell damage boost (from Earth and Moon).



Play the toons which will be the most fun for you.
If you have a level 80, you can do a faction transfer (or even a server transfer, if you prefer) and take quite a bit of gold with you... I believe 20k or so.




I would go with the same aspect ratio.
The size doesn't matter so much, but you want the ratio's to match for faster switching of toons.

I'd probably divide the slave monitor into quarters, which gets you three filled areas and one area for a wowhead/thottbot window.
Then stick the main on the main monitor.
But do whatever works for you.




If you go with a booster, then instances.
If you don't go with a booster, then quest.

You can do pvp along the way too, whichever route you like.
It's a change of pace, and gets you some experience too.