OP, I was accelerated by 2 years. Looking back it was the worst of all worlds. The work still wasn't a stretch so I remained
bored, and didn't internalise any kind of work ethic. And socially and emotionally it was an unmitigated disaster. I took 2 years out between A levels and university but was still socially unready for uni and ill at ease at 18.
I would have been very happy at your school for Hermione Graingers, but it doesn't exist. In the real world I tend to think the least worst option for an out of sync child is to focus on social and emotional literacy at school in the early years, and create space at home for delving into the nerdier enthusiasms. Mixed age group clubs like Scouting are good once they're a bit older. Music or sports, to give a social connection point outside academic interests, are well worth encouraging if he has any spark of enthusiasm. Also plenty of opportunities to try things that don't come easily and foster the skills of perseverance, and how to fail and get back up again (a much harder lesson in your 20s or 30s...).
Academically, I don't know the answer. In a small school it is most unlikely there will be an age peer operating as far ahead as your son. But that would probably be the same in a large primary too. Going up a year might work fine socially for your DS as he is autumn born but there is likely to be systemic reluctance.
Unlike your son my DS is just bog standard run of the mill bright. Even so, he currently refuses to engage with the maths activities in Yr R because apparently "it's for babies". The teacher has yet to twig he can do even the necessary basics required for Year R numeracy but isn't worried because he's got the whole year to master it (sound familiar?). So if that is an issue for a regular bright child, of whom there will be at least a few in every classroom, you are right to be wary of your much more able son flying below the radar, and of possible disengagement. But I think all you can do is watch and wait and respond to whatever happens, and support him to manage his feelings about school as best you can. I'm sorry if that's a gloomy message.
In terms of books, my DS (also 4) isn't an early reader so we don't have that problem, but he's a good listener with a wide vocabulary and we've had similar content-related issues with selecting age appropriate chapter books to read aloud. (He will listen to a chapter book in a single sitting if we have the stamina, which is a whole other problem!). DS recommends Roald Dahl, The Magic Faraway Tree and its sequels (I overcame my Enid Blyton aversion), the Carbonel series by Barbara Sleigh, the "Jolley Rogers" books, Dave Pigeon, E Nesbit's "Five Children and It", Swallows and Amazons, and the Jo Simmons Pip Street mysteries, all of which have been fine as regards content (and he's quite a sensitive child). School have got the measure of his interests and helped him pick some good ones from the library, for reading aloud at home, which is nice. The story dice mentioned upthread are great, too - someone gave DS a set last Christmas and it was a surprise hit. And he dearly loves an Usborne factual lift the flap book, or one of the DK Eyewitness series, if it chimes with a current interest.