Sound like the clutch is not disengaging but why only 4th gear, rather odd if that is the case.
Do you still have your hand on the shifter, applying pressure towards 4th, when recycling the clutch?
Or perhaps not realizing you are releasing tension on the shifter before the clutch is all the way out?
Does it do it all the time or on occasion, you will need to replicate it for the dealer so best to either be driving with one of their people in the car or at least be riding in it.
In the mean time if not already done take it out and focus on making a solid shift, clutch in all the way, shifter all the way into position and hold it there, release clutch all the way, add more throttle input (try it while releasing the clutch and after it so see if any difference) If the problem goes away then you will need to shift more deliberately but also there could still be a bit of an issue with the car.