Hey there, I just picked up my new to me 91 K100RS and I was putting it around town and notice a loss of "power" or falls flat after 4000 RPM in every gear. it still accelerates but not at the same rate.

Im not really sure where to start. I did search the forum but didn't find much about it except it could either be a throttle sync or maybe a fuel filter clog? any help or direction would be much appreciated!

Old, stale fuel? Clogged air or fuel filter? What are their history? The bike is new to you. Try a really good fuel system cleaner like Seafoam, and a few more miles to allow it to work.

Poor spark plug and/or ignition lead condition, misadjusted ignition timing or even a failing coil?
What have you attempted thus far? Can you check lead resistance? ~5K Ohms.

There are many things you should go over that might remedy the issue, or at least lead you to something after attempting to eliminate some basics. I would not go near the throttle body adjustment screws without a real understanding of their function and before I'd tried every step in troubleshooting a "loss of power" perception.