What Causes The RPM's To Get Stuck?
 

The REV light was adjusted to 4600 on this, but the needle wont go past 4K

I'd say your tach is fried, obviously it's not showing the correct values and that huge stutter it makes midway through usually means the stepper motor is going out. HOWEVER you can verify to make sure it's not a bad signal being sent to it by using an OBD2 scanner and reading the tach values from there, if it shows the same values as displayed on the tach then you have some kind of issue with the signal sent from the ECU.

It's also possible you have a ground issue with the combination meter but that would usually affect EVERY gauge not just one.

They can but I have no idea how you would go about doing that on this cluster. Anything can be replaced if you have the necessary skills :)

There's a gauge cluster diagnostic routine detailed in the service manual that'll let you move all the gauges, test the lights, etc. It's intended for exactly this kind of thing. It might shed some light on this.

The problem with the diagnostic mode iirc is the gauges only go up like 25% when you press the trip odometer button. I can look for the instructions on it later when I'm home but I think you just hold the odometer button down and turn the ignition to ON then press the odometer button 3 times. Then every time you press the button again it makes the gauges move.