You may be on to something with a spark plug that is oil fouled. I would be guarded about putting too much stock in it being a valve guide seal however. Typically, when valve guide seals start giving problems, you see it on more, if not all, cylinders. Typically they fail from age, and equally typically, they are the same age. Also, based on that, valve guide wear could be contributing as well, but again, most likely indicated on more than just one cylinder.
You did mention compression on a cylinder of 160, but you didn't clarify if that was all 4 cylinders. If only one spark plug is coming out oily black, it would be more common to be related to a ring or cylinder wall problem. Since you have to remove the head to check the valve guides and seals anyhow, I would suggest removing the cylinders to inspect the affected piston, rings and cylinder while you have the head off. If nothing is wrong, the only additional part you are risking is a base gasket, which is relatively inexpensive. If the bike is high mileage, at the very least you could replace the rings and help the overall performance of the bike.
Bear in mind that you really need to get the carbs synced to ensure the bike is running properly. Any motorcycle shop should be able to sync them for you, and most shops charge about 1 hour labor for that job.
You have thrown out a lot of variables here so it is difficult to give you specific advise. it is confusing that in your original post you said it has good power/torque up to 3500 RPMs, then in your last post it revs higher but only on 3 cylinders.
Before you take it apart, if it truly is a problem of the spark plug oil fouling, which is what it is called for what you described, replacing the affected spark plug will allow the bike to run correctly for a short time until the oil fouls the new plug. One spark plug should cost you less than $5 and it is the cheapest easy diagnostic tool at your disposal now. If I were in your shoes, I would definitely replace the spark plug and see if it runs normally on all 4 cylinders. If it doesn't, there is still something else wrong with the bike. To reiterate, replacing the one apparent bad spark plug with a new plug is for the purposes of testing and eliminating variables. It is not intended to be a fix for the symptoms, we are just trying to affect the symptoms.
Depending on how severe the problem is about oil getting to the spark plug, the new plug could last a short time or quite awhile. The important point is, it should last long enough to verify that was the real problem or not.
If you have any more questions, feel free to ask.
PS, as far as my experience, I went to the American Motorcycle Institute and became a certified motorcycle technician in 1976, and since then have worked in motorcycle dealers continuously since 1984 as everything from a line technician, service manager, general manager of a franchised dealer, a factory service road rep for Yamaha Motor Corp and owned my own motorcycle repair shop/retail store. I have worked in dealers and have factory training in Suzuki, Yamaha, BMW, Kawasaki,Honda, KTM, Polaris and Eton. In addition to that, during my time with Yamaha, I was signing the certificates for Yamaha dealer technicians verifying they had successfully completed Yamaha technical training. With Yamaha, I as personally certified in and trained techs on the service and repair of Motorcycles, ATV's, Side by sides, Scooters, Outdoor Power Equipment, Personal Water Craft and Jet Boats.