OK I did some comparison between SVP3 and SVP4 on 1080p videos.
SVP4 has higher usage both on the CPU and GPU, because its settings are higher.
One of the settings that makes the most difference is Motion Vector Precision being set to half-pixel instead of 1 pixel.
I had adjusted SVP3's settings so that it plays smoothly consistently for the various types of videos I'm playing. How does SVP4 manage its auto settings? Are the settings fixed per resolution or variable? Because the settings I set in madVR (fixed) depend on what is configured in SVP, yet for SVP, it has to be configured according to the "lowest common denominator" where if one video causes lag, settings had to be decreased for all videos of that resolution.