I have had Dell 30"s in many spatial combinations ( 2 horizontal, 2 vertical, 3 horizontal - sides turned in, 4 in inverted T) and I have found it always better to be able to put my primary information front and centre, with lesser info to the sides.
4K gives you twice as many pixels as a 30", so if you are satisfied with the number of pixels on your two 30"s, then 4K will be OK. But the ratio is 16:9 rather than your current 32:10, which may serve you better if you are using the arranger and score windows over each other.
However, while doing some php programming, my wife noticed that I still had a lot of overlapping windows, so I joked that I obviously needed to get another 4K for each side of me! There was a reason I had 4 x 30"!