For anyone wondering like me:

> What is the 4-2-6 breathing technique?

> The 4-2-6 breathing technique is a calming exercise. First, inhale slowly for four seconds. Then, hold your breath for two seconds. Finally, exhale slowly and steadily for six seconds. This technique helps by making your exhale longer than your inhale, which is a signal to your body to relax. It's particularly useful when you need to settle your mind before sleep or if you're feeling anxious and need to steady your nerves.

Source: https://www.calm.com/blog/breathing-exercises-for-anxiety

And there's nothing magical about that set of numbers. I learned it as 5-5-5-2, but in the end it's about using a pattern to regulate your breathing, and - I believe - to force your attention to counting breaths, an anxiety-free exercise, allowing it to lower its hyper-vigilant awake state.