Microsoft claimed that the next gen of Surface Pro is compatible with SP3 docking station. With that in mind, I think the thickness of SP4 would possibly be the same. Technically the SP3 docking station can certainly hold a thinner device, but it will be quite odd that some space is left empty in the docking slot.
But I still support a thinner Surface Pro. Microsoft needs an even thinner Surface to fight against iPad Pro.
Or thinner bezel? If yes, it will look super cool.