So, I heard that we can break in a sponge either by using boosters or by playing with the rubber long enough... If I use a booster that lasts for let's say 4 weeks, would the rubber, after 4 weeks of use need booster again, or are 4 weeks enough to break in it?
There is 2 elements to breaking in.
one is the sponge and yes, booster does help with it.
the other is top sheet and booster does not help with it.
Breaking in the rubber can also attribute to testing the rubber, testing the glue job etc.
There could be times where the rubber is just not right and maybe a fresh gluing could be required.
Majority of the time, is to just getting used to the new rubber.
After a few hits, the topsheet will loosen up a bit, and allow for more grip.
You would require impact on the topsheet though and the time varies. Most are fine with a or two session of (high level) training.