"low index for 4k" is different from "videos of 720 through 2160p resolutions ... playing at 24fps"
if there's no errors in the log and SVP index is ~1.0 then it may indicate the old issue with 30xx series
edit %APPDATA%\SVP4\override.js
insert line:
smooth.nvof.cost = 0.7;restart SVP
see if something changed; try different values below 1.0 - 0.8; 0.9.