This information applies to versions: 6
Nik 6 Perspective offers additional features which were not included in the former Perspective EFEX: ReShape, Flipping images, automatic perspective correction preview, guidelines
There are no differences between DxO ViewPoint 4 and Nik 6 Perspective in terms of functions or features.
The unique advantage of DxO ViewPoint is its ability to be used seamlessly as a plugin within DxO PhotoLab, whereas the extra benefit of Nik Perspective is that it can be installed alongside the other Nik plugins, making Nik Collection by DxO a truly comprehensive suite of plugins.