Is that how it works though?
My impression is that the current interest would get boosted by 10%, meaning that if somebody had 4% interest and that interest amount gets boosted by 10%, meaning they would be getting 4.4% interest.
Maybe I'm wrong, and if somebody is getting 4% now and they get 14% after, then that would be another story, but that seems way too high to be realistic.
If it is option #1, then I wouldn't say that there is a big difference between 4% and 4.4% interest for example.
When your interest gets boosted by 10%, thanks to the premium membership, your annually interest will indeed be 4.4%. Not a major difference, but on a large amount, this will give you a nice extra.
In a lot of countries, you will not get 4% interest on your fixed deposits or other investments and it will also not be paid back on a daily basis.

Exactly, in Belgium, the interest in your bank account is around 0.11%

. So getting 4% is good and it is seems to be sustainable for freebitco.in also.