I don't have any experience with that card, but the description states that it has four "discrete" DP outputs. This suggests to me that each port has it's own DP bus. That in turn means if you have only one monitor connected to each port, that monitor gets the full throughput of a DisplayPort 1.2 connection for itself. That would mean adding a 4K monitor to that card should not be a problem.
This conclusion is supported by the fact that you would not be able to run three 30" monitors on a single DP bus. It only has enough throughput for two. Yet you're currently using three such monitors.
So I'd say skip ordering an additional card unless it turns out my guess is wrong.