My 4 year old sounds similar.
He’s underweight so told he needs to eat more, yet has zero motivation for food. He isn’t bothered at all if he didn’t eat all day. Hes up and down from table every though dh and I sit at the table with him and eat at every meal, have done since weaning. He’s in Tripp trapp seat at child mode or can sit on main soft comfy bench if he prefers. We can’t just say sit or don’t eat, as he will happily take the don’t eat option.
We have recently started tv at the table which I know they say is bad, but he actually sits better than he ever has. Still messes around, but has eaten more in the last 2 months of doing it that the last 4 years. We use the iPad, and it’s not even kids shows but stuff we can all watch and talk about. Master chef, Simon reeves travel shows, Ben fogle etc. It keeps him engaged.
He also takes forever to go to bed. Yesterday had 4 hours forest school, swam 90mins, park with friends after, helped in the garden. Took him to bed at 7.45pm, and he finally fell asleep at 9.30pm. Regardless of bedtime he always takes min 1hr to sleep, often 2+hrs is we take him up too early before he’s tired.
strapping into a highchair at 4 is laughable imo. Mine would and has just unclip led straps and climbed out by 18-24months. He used to be able to unclip car seat really early also, maxi cosi told us it wasn’t possible for a 1-2 year old but it 100% was