These are already in the Zhengzhou hotspot
Luoyang would be the better hotspot.
0) 3 Sites are directly in Luoyang. 1 other is closest to Luoyang. As good as hotspots get.
1) More history. It's the historic capital of China.
2) Xian and Pingyao can also be added. Potentially even more.
Problem I see is Chinese high speed. Essentially, you can make loads of places hotspots by virtue of having bullet trains. Pingyao hotspot would include Beijing :D
So my preference for hotspots in densely covered areas is:
1) Pick a center of gravity that has historic significance and it's own WHS.
2) Add connected WHS if they aren't well covered by other hotspots.
3) Note down related hotspots.
4) Relax the 4 sites criterium IF there are local sites.
E.g. assuming Xian get City Walls added (100% proponent myself), it has 3 sites on its own (silk road, mausoleum, city walls). This to me would be a valid hotspot on its own even though it's below the 4 site threshold. Luoyang already meets the 4 sites threshold.