Why monitors must be in exact pairs on digital video cards
The new digital video cards are designed a bit differently when it comes to connecting monitors to them, particularly when connected with DVI or DisplayPort. This pertains to multi display video cards and systems.
Monitors must be paired meaning the same exact make, model and resolution on ports 1/2. On ports 3/4 they must also be exact pairs, but they can be different monitors on subsequent pairs.
For example, you can have two identical Asus monitors with the same resolution settings on ports 1 & 2, and two identical Samsung monitors with the same resolution on ports 3 & 4. In other words, they can be different PAIRS. You could NOT connect an Asus and a Samsung next to each other on ports 1 & 2.
If you have all 4 of the same exact monitors, ignore this requirement you are fine.
This applies to all digital video cards regardless of brand. There is one exception. If you are using a video card with DisplayPorts as well as ACTIVE video adapters. If so you can mix and match any make model monitors you like.
The exception there would be if you are using extremely old monitors and pairing with a new montior. This is too much of a spread and could cause display issues even when using at Active adapter as mentioned above. The solution there would be to consider buying newer monitors.