Hi @Nick1866 ,
After getting our first (and so far only) 4-star review we decided to directly ask guests to (i) write a review and (ii) give a 5-star rating if we met (if not exceeded) their expectations. I would already reach out to them after check out to ask how everything went and ask for their feedback (as we genuinely want to continue improving). In the beginning I was a bit embarrassed to do that, but so far we got a positive response (50+ 5-star reviews after that 4-star one).
Basically, we got this 4-star overall rating (even though they gave 5 stars for all six individual feedback items and a positive public review) because they thought they should compare our house to a hotel. As it definitely doesn't provide a 5 star hotel experience and they did not know 4 was a low rating, they thought they were giving a positive review. It is why I don't think explaining to guests what less than 5 stars mean is necessary a problem or goes agains Airbnb's guidelines.
I personally don't like the idea of a sign because it becomes part of the decor. So maybe adding a rating guide to your welcome guide might be better or directly writing to guest after their stay as I do. Airbnb has a schedule message tool, so you can automatically send a personalized message (ie. including guest's first name) after each check out.
The only thing is that "minor issues" should not low a score by 1 point in my opinion. So, you might be incentivizing more critical guest to give you 4 stars as it is almost impossible to have a completely perfect stay. I'm not sure about the system @Branka-and-Silvia0 is mentioning as I see both "more than expected" and "as expected" as 5-stars when I'm rating my stays as a guest.
Hope it helps. Good luck!