does someone know why there is a {}{4} and {}{2} ?

Yes, these are annotations from the very first version of Tatoeba. It was used to "link" words to each other in the different languages. Basically, it means (for instance) that the equivalent of "thoughts" in German is "Gedanken".

The {}{number} were converted into some javascript so that when you move your mouse over a word, all the words in the other sentences that are linked to it are highlighted.

I should take these annotation out because they are not used anymore, but I haven't had time for that. And they're not really harmful.

in fact i would have found this feature useful , or at least have the possibility to enable/disable it , after maybe you've delete this feature for some good reasons

It's disabled only temporarily. In a few months (hopefully) I'll be working on a way to process that automatically.
At the moment I don't want people to focus on this task (because these brackets would have to be added manually). There's already a lot of work to do on translating/correcting the sentences, and that is a task a machine cannot do so well...