UPDATE:
When (hand)scrolling in Reaper it's the process csrss.exe which uses the GPU. That happens only in Reaper though, when scrolling around in other programs like chrome or explorer csrss.exe is around 2-3%.
And the lag gets better when Reaper is not opened full screen on the 4k monitor. When resizing the Reaper window to ~25% it is nearly as responsive as on the smaller monitor.
Also when setting the 4k monitor down to 1680x1050 resolution, the lag is gone and it is as snappy as on the smaller monitor.
That could lead to the conclusion that the lag is related also to the max displayed track count, cause on 4k I see around 80 tracks at the same time, and on 1920x1200 I see around 40 (on top of that it also depends on the max item count displayed on the screen as seen in video 1a).
EDIT:
Tried to hide also most of the tracks using the track manager, but that does not change anything when resizing the remaining tracks to fill the screen it seems. So not even the track count is the culprit, it seems Reaper just struggles in general on 4k when the full screen is used...