sorry, thats not accurate…
if you just ADD an ADDITIONAL disk - there will be no rebuild - so the “several days” is completely irrelevant.
its only if you were to REPLACE a disk it would need to rebuild the missing data on the disk you removed
simply adding a 4th drive does not involve a rebuild - it will be immediately available and all of your drive lights should pretty much immediately go SOLID green (rebuild = flashing yellow/green).
in order to take advantage of the additional space/speed fully it will (probably) RELAYOUT the data (but this is differnet from a rebuild - at no point will you ever be “at risk” as you are during a rebuild), but it does this totally silently, it wont tell you its doing it, and you wont know when it finishes it. It wont stop your mac or drobo from shutting down/going into standby either, it will just start doing it again when it comes back online.
In fact, due to its black box nature, we cant even be sure it does this - it may leave that 4th drive empty until you start adding more data to drobo - and then do the relayout as part of adding the data, although i suspect this would be ineffiecient so it may not do this, but we just dont know
Also DRI do advise that if you wanted to benchmark your drobo you give it a few days to “settle down” after you add/remove drives and get back to s safe state, so its safe to assume it does optimise the data layout in the background, but again, you never really know…