Not super useful for you now that you have the new machine, but if your old MBP is the Mid-2014 15" model, it can drive up to two 4K displays at 60Hz, but only

Not super useful for you now that you have the new machine, but if your old MBP is the Mid-2014 15" model, it can drive up to two 4K displays at 60Hz, but only over DisplayPort. I have the same laptop and recently upgraded to a pair of 4K displays, and to get 60Hz I had to use mini-DP-to-DP cables rather than HDMI.