Is it possible to use three csv-5400 with AMD FirePro W8100 (which support four 4K DP output) to setup a 6×2 video wall (3 MST and each 2×2) ?
The AMD FirePro W8100 can support a maximum of 6 displays. If you want a total of 6 displays on this card you will need the MST 1-3 CSV-5300A.
For example, if you use a CSV-5400 on Port 1 and use 4x 1080p displays you will be able to use port 2 (single display) and 3 (single display) to make a total of 6 displays.
On a different topic about 4K;
AMD allows on this card to configure up to 6 displays in 4K at 30Hz, for this you need to address them how it is configured. You can read here under Future-ready 4K