Year 3 or 4. Is that analogous to US 3rd-4th grade (so 8-9yos), or US 2nd-3rd grade (7-8yos)?
English school (Scotland is different) goes Reception, Yr1, Yr2,... and children start Reception the September they're 4. (Yes, it's crazy - some children do their pre-start settling-in visit while still 3. Scotland's cut off is a few months later so the children can't be quite so young when they start. Reception is supposed to be still child-led and play-based, but whether it is so varies; in reality, most do learn to read and write that year. In England your child does not have to be educated until they are 5, but you can't start them in Reception a year late - if you don't send your summer-birthday just-4yo to Reception, your maximal delay alternative is to have your just-5yo enter Yr1.) So Yr3 children are 7 when they start in September and will turn 8 before they start Yr4, Yr4 children are 8 and will turn 9. Not quite sure how to translate that - you have K, 1st, 2nd,... so it's tempting to match up K=Reception, 1=1st,...; but at least some US children seem to start K later than English children start Reception. So I guess the answer is "it's in between".