It’s not really a thing. ADHD is usually medicated with stimulants, not depressants. Counterintuitively, ADHD brains suffer from a lack of sensitivity to dopamine and similar chemicals, and stimulants flood the brain with these giving ADHD people an experience closer to neurotypical.
Alcohol as a depressant does the exact opposite thing that an ADHD brain needs, and will make core symptoms worse.
It’s just old fashioned terminology. 4% was the strength of a standard bitter or lager, round about 5% a premium bitter or lager.
The UK doesn’t use the term “light beer”, so you can probably just think of it as being the equivalent to that distinction.
Obviously there are some wildly strong craft beers out there these days, but the lingo still is what it is.