That's odd. Right now I'm using two different desktops where one is an eyefinity group (4800x2560 and 1600x1200).
Just thought I would update this that it looks like it was a driver issue. With 10.12 it just ignored it everytime I added a display as 'extended' but when upgrading to 11.2 (my sister was having some issues with that driver hang issue and 10 sec lag spike so updated drivers in an attempt to fix) I was finally able to have a 2x2 display group (3840x2400) + 1920x1080 as extended desktop. This was important as she is using my vp2290b and the 4 inputs are going to a single monitor.
Once i verified this my dad just ordered an eyefinity6 card as well. He is tripple monitor at work and also runs a vp2290b so needs 4 of the outupts for the vp2290b, a dell 3007 WFP and a dell 2407 WFP.
Newegg has a really good deal right now on the 5870 eyefinity 6 as its only $279. Also I found a seller on ebay that sells OEM active DP -> Dual link DVI adapters for around $50 shipped which is a lot cheaper than the $100+ they are from everywhere else I see them being sold (My dad needed one for his dell 3007wfp).