I need you to help me with this question. In tournaments with reebuy you can leave the table when you run out of chips for example at level 4 and re-enter the addon period .. or once you do not buy back at the moment you are out.
It would be something like playing at the beginning of the tournament and then being out a few levels and entering for the end. Please someone take this doubt out of me! Thank you

If you run out of chips during the registration period, you are given the option to rebuy. If you choose not to rebuy, the tournament is over for you and you cannot rejoin the tournament even if the registration period is still open.

Hope that answers your question?

Re-entry is the same as rebuy only with a rake, if you lose, you can re-enter the tournament at any time while registration is in progress.
Although there is one caveat, in tournaments with re-entry, when you re-enter, you are registered as a new player, for example, 380 players are registered in the tournament, you are knocked out, you make a re-entry and 381 players will be registered in the tournament, you are put at another table, at another place.
With rebays, the number of registered players with re-entry does not increase, and you remain at the same table and the same place.

As the others have said, most if not all poker sites only allow you to play the same tournament once. If rebuy or reenter is an option, that offer is typically open for around 10 seconds, and if you dont take it, the tournament is over for you. If you try to play the same tournament again later, you will get a message like "sorry but you have already played this tournament". As for R+A tournaments there are two strategies, which are both reasonable:

a) Enter early and rebuy as many times, as it takes, and then purchase the add-on
b) Enter just before late registration close, try to make it to the add-on without rebuying and then purchase the add-on

The worst strategy in R+A events is to enter early and not rebuy. This is something, I only do, if something happened during late registration, which made me change my mind. Maybe I am starting to feel tired, or something came up, which makes me want to end my session earlier. Or I made a deep run in another tournament and want to focus my full attention on that one. Or my table is simply really bad, and I am beginning to tilt after making multible rebuys. But when I enter a R+A my first goal is always to make it to the add-on.

When you lose in a tournament you are offered to make a reentr if you refuse to enter the tournament you will no longer be able to.