190E 2.6 fourth gear slipping
 

I daily drive a w201 and yesterday on the way home the trans went to shift into fourth and just revs to the moon. It feels like a slipping clutch really. I can easily pop it back into third and drive, but it isn't very practical. I've read that some people have had luck just flushing the fluid and putting in a solvent to clean out the valve body. I'm at my wits end with it honestly at this point. While the pan is off is there anything I should be looking for that might be causing it? I'm not knowledgeable with automatic transmissions and have always assumed they run on vacuum and black magic.

Any help would be greatly appreciated. I do really well with photos and everything I've seen that has photos has broken links.

Thank you in advance.