I used to sit on a Google / Alphabet promo committee (more than half a decade ago, so things have changed a bit). The bar for L3 (newgrad) to L4 promotions is not high. It's certainly not a rubber stamp, but we were looking for:
- Gets things done fast enough with high quality
- Works mostly independently
- Has recommendations from more senior peers who can speak to what they've done
- Is starting to show several of the expected marks of a Senior (L5)
I had at least one, maybe two new grads who I mentored get promoted to L4 within a year of starting full time; they had both done internships at Google so they didn't have as much ramp up time.