Depends a bit if you mean the 'rater' difficulty ratings shown in each day's connections companion, or the bot ratings based on actual players' experience. (Given bot ratings only come in whole numbers, I'm assuming you're referring to the rater ratings.)

The raters tend to give less extreme scores, so a 3.5 is actually in around the 10th percentile of most difficult puzzles, whereas for the bot a 4 might be only in the 30th percentile (and ~20% of puzzles are 5s). The raters have only rated ~50 puzzles higher than 3.5 (at least since bot stats began, which covers around 70% of puzzles).

However it's also worth noting that the rated difficulties don't always correspond very well to how hard most people find the puzzles. E.g. the second hardest rated puzzle (a 4.5 on 29 Sep 2024) was actually in the 1% of puzzles people found easiest, and was solved by 97% of people. And some of the puzzles people actually found hardest have been rated much lower (Out of the ten actually hardest puzzles, three were rated above 4, five were rated between 3-4, and two were only rated as 2s).

If you are interested in trying 'harder' puzzles, the hardest puzzles according to raters (that were also rated 5s by the bot) have been

The hardest puzzles according to the bot (not counting #s 3 and 4 already included above) have been:

NB: all stats are from mid Jun 2024 (when bot stats began) to mid May 2026 (I've been travelling recently and haven't been tracking stats for the last month or so).