Multiple external displays for 7420

I'm looking at getting one of the new 7420 laptops but I'm trying to figure out how many (and what resolution) monitors it can drive?

Ideally I'd to have a couple of 4K monitors in the mix. If a dock like the WD19TB is necessary for this, is it important that the monitors support DisplayPort 1.4? (I was looking at the S2721Q but noticed that only has DP1.2)