Good thing you highlighted "4th gear" and "40km/h"

Blind shot in the dark.

You most probably have a bad batch of fuel or blocked filter. or just not using 95 octane.
Its a phenomenon called "pinging".
Its the point where fuel is incorrectly combusted in the engine, which in turns, tries to turn your crank the opposite way.


Could be something else though. Im just pointing out the most common thing.

40km/h in 4th gear is abit too much load. Id try downshifting to 3rd if i was going that slow.