How to navigate reorg & changing priorities as a new hire?

Sorry to hear about the re-org: This is going to happen all the time at Meta and pretty much every other FAANG company.

Ideally, you shouldn't have to learn this as a fresh E4, but Meta, especially now, is a "sink or swim" environment. If you don't have any major impact shipped come PSC time, it doesn't matter how many excuses (even valid ones) you have. You absolutely need to find scope. There's 2 main ways to do this:

Ask Your Manager

You always try this first, especially as a new E4. It's easy for managers to lose track of their reports, like not realizing an entire engineer has nothing to do. From my understanding, your manager is figuring this out alongside the rest of leadership, but that doesn't mean you should just sit back and wait. Keep pressuring your manager for meaningful work. Managers make and break promises all the time - The onus is on you to keep the thread alive.

Create Your Own Scope

This is more something for E4s who are closer to E5, but the Meta bar is higher now, and there's no reason why you can't develop this behavior earlier. Follow the advice from here for starters: "How do I come up with innovative, impactful ideas and bring them to my team?"

You also probably don't need to start from scratch here either. Most teams should have some semblance of a backlog, particularly with "Better Engineering" type tasks. Go through those. On top of that, talk to other engineers on your team to see if they have some backlogged tasks they can share with you.