TL;DR: I would use up the 4*, petrol is too expensive to waste!
Cheap supermarket petrol from the 90's could evaporate in the carbs and leave a sticky deposit, however, if you had a full tank of 4* BP from 1998 in your tank for the past 20 years then the likelihood is that it's perfectly fine. Petrol in a sealed container or with a small volume breather will last indefinitely. Does it lose its power? Well I guess if ethanol was present it would evaporate far faster than petroleum would and so reduce the RON ever so slightly, doubt it would be noticeable. The jelly thing, you're entire tank wouldn't turn into jelly but some may form, caused by ethanol separation from the petroleum and because ethanol is hygroscopic it draws in water from the air and you end up with a squishy jelly like gunk either floating about in the tank or left behind in a carb bowl.
Edit: Couldn't leave it there with my pseudoscience opinion, went and looked it up. First document I hit was from BP who says petrol will only last about 3 weeks without fresh needing to be added, I don't actually fill my tank up that often, lummy...
https://www.bp.com/content/dam/bp-count ... -tanks.pdf
https://www.bp.com/content/dam/bp-count ... ndling.pdf