How far away do portals need to be so that they don't go to the same place?

I'm playing on a SMP server and am trying to use the Nether to link up all my disparate bases around the world. Note in the below, A is Overworld, B is Nether.

I started with one portal (A1, thus spawning B1 in the Nether), and then created another portal a ways away in the Nether (B2), hoping it would be near the main area all the other players congregate. It (A2) was pretty close to where I wanted it, but not exactly. I then created another portal a short distance away (A3), which was where I was hoping A2 would land. I then went somewhere in the neighborhood of 400 squares away from where I initially built A1 (in the opposite direction of A2/A3) and created another portal, A4.

One would think A3 and A4 would create B3 and B4 on the Nether side, respectively. However, A4 linked to B1, and A3 linked to B2. The weird part is, A1 is further from A4 than A1 is from A3.

I hope that made any sense. My question is:

How far away do portals on the Overworld map need to be from each other in order to spawn separate portals in the Nether? Or, what are some ways to ensure a new portal won't link to an existing one?

Just make sure there's at least a 1025 blocks distance between your overworld portals:

Likelihood of 2 overworld portals linking to the same nether portal - Normal World portals that are within 1024 distance of each other on either X or Z axis are almost always going to link to the same Nether realm portal on initial construction because 1024 translates to a distance of 128 in the Nether Realm, and the game checks for existing Portals within 128 "radius" around the destination (the 257x257x128 box).

(Minecraft Wiki)

You should also note that, in SMP prior to 1.6, portals were bugged for quite some time and needed server mods like Bukkit in order to work. Ask your admins/operators, there may still be some Bukkit plugin running on your server handling the Nether.

How far away do portals on the Overworld map need to be from each other in order to spawn separate portals in the Nether?

If you want portals spawned in the Nether, then lunbook's answer is AFAIK correct and you'll need 1,024 meters (perhaps more due to rounding) between overworld portals to get you a brand new Nether portal.

Or, what are some ways to ensure a new portal won't link to an existing one?

Just because the game won't provide you a new Nether portal within this radius, it doesn't mean the game won't use a better portal if it can find one. So in general what you want to do for every portal after the second is:

1. Build your overworld portal.
2. Light it up.
3. Before entering, press F3 and note down the X and Z coordinates. For example, say I want to make a portal to some underground, underwater stronghold I found while boating around. F3 reads: X: -418, Z: -163 (don't worry about decimals).
4. Go through.
5. You should (hopefully!) be at your old portal. You can verify this by going back and forth through it quickly.
6. Whip out a calculator and divide both coordinates by eight. In my example, −418/8 = −52 and −163/8 = −20 (discard the decimals, but make sure you keep the sign.)
7. Reach that place with the F3 screen. In my example I mined my way to X: -52, Z: -20.
8. Build a portal there and apply fire to it.
9. Try it and, if you didn't mess up, you should be good to go!

What you need to prepare for is if you actually do get a new portal at step 5. That's actually the worse scenario, as now you have to live with the automatically generated portal placement (which might even be encased in Netherrack in all directions!) and somewhow trace back to your existing portals. You should keep a list of notable portals' coordinates noted down to save the long overworld trip back home. A nether map may help, but the constantly rotating cursor really does get in the way

Every 1 block in the nether is 8 blocks in the real world. So, say if you want to travel 800 blocks north, build another portal 100 blocks north in the nether from your first portal.

does not work in xbox

been doing some testing on xbox working on superflat world so terrain not a issue looks like so far every block in the nether is only 3 maybe 3.5 blocks in overworld built few portals and if u divided by 3 seems to work out

and it also appears portals have to be 129 overworld blocks away from each other (or 43 nether blocks away) i think i got it fiugerd out :) but still testing this theroy