You're not going to find a 4-slot x16 mobo, for Z170/Z270/Z370, without using a PLX chip. And someone bought that company, and apparently, they are now too expensive to use on mobos, at least the ones that you'll find under $400. At which point, you might as well go with the X299 platform and Skylake-X, if you really need that many PCI-E lanes. The mainstream CPU/chipset, only supports 16 CPU PCI-E 3.0 lanes, and 24 chipset lanes, which are funneled down to 4 lanes, where it meets the CPU and memory controller.
TL;DR - You're buying the wrong platform. Think bigger.