I was dealing with this weird youtube problem, on my huge display youtube displays 3 huge thumnails per row. In order to fix this you can use this css code below.
You can use this with something like tamper monkey, Stylebot, or like me ARC's Boost feature.
ytd-rich-grid-renderer {
--ytd-rich-grid-items-per-row:4 !important;
}
arc
, boost
, cpu-benchmark
, user-scripts