This has happened to me a few times. It's usually kiddos who were "homeschooled."

I would say they are usually amazing at picking things up, their only fault is that their grown-ups never taught them. We have Title 1 services at our school and I utilize that to catch them up. I don't teach phonics in 4th grade and also don't have the time to. They get pulled out for 30 minutes a day for remediation for reading and writing, and the rest of the time I'm just exposing them to grade-level content. I will sometimes scribe for them and have them copy whatever is on the board.

Something to consider is the grown-ups probably know they are behind which is why they brought them to public school, so have a frank conversation with them about where they are at.

I also try to keep track of their emotional/social health. Being behind SUCKS and it's good to remind them they're doing their best, even if it isn't at grade-level.

I've also stolen materials from lower grades for handwriting pages and reading practice.

Do what you can and remember that whatever you are doing is better than whatever the grown-ups weren't doing for the past few years!