The ES (early start) of the first activity in the path is 1. The EF (early finish) of any task is its ES plus its duration minus one. So start with Activity A. It's the first in the path, so ES = 1, and EF = 1 + 6 - 1 = 6. Now move forward to the next activity in the path, which is Activity B in this diagram.
or
Look at this free paper on the topic examples.oreilly.com/9780596102340/hfpmp...errata_pp257-265.pdf

so if es at 4 the formula will be EF=(ES+Dur)+4?

Both are correct. It is just that if you start on Day 0, then what you have shown is the way. If you start with 1 as Day 1, then the formula is different.

Starting with 1 as day 1. Then for an activity going forward, EF = (ES + Dur) + 1. The successor activity B will have ES as the next day after the EF of activity A. The backward pass to find LS = (LF - Dur) - 1.

Please watch Aileen Ellis' videos on youtube. She is very clear and professional in her explanation and you can learn forward pass, backward pass, float and free float in 1 video with a clear cut detailed example.

Rita's book does the calculations exactly like this example:

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Meaning, if activity A starts at 0 days and takes 3 days, then the EF = 3. Activity B happens immediately after, so Activity B's ES = 3.

However, I've seen some other videos and the PMP book say that Activity B's ES has to be 4, because Activity A finished at the end of day 3, so B has to begin at the start of the next day, which is day 4.

Which one is correct? Your critical path duration will have different values depending on which method you use. I hope this was clear.