99MBSport

I am trying to get the shifter on my C280 Sport (1999) to go into 4th from the Dr. position. It won't budge. Took top cover off and it looks like a sealed unit. If I take the entire shift mech out can the internal parts be seen or do I need to change the entire assy. Other than not going into 4/3/2/1 with the lever it runs great, all speeds and shifts normally. Any answers would be greatly appreciated.