Mercedes E320 2004 Gear problem

  mercedes/e-class/2003-2009

Hi,
My Mercedes E320 2004, while driving and when it shifts from 3rd to 4th gear the RPM rises and I hear the motor sound for about 1 second and then everything becomes normal.
What do you thing the problem is, and is it fixable without changing the gearbox?

Thank you for your asistance

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