if yesterday´s level was 2 and today I make it 3 if you like then is 3 bigger than 2? I would say yes for obvious reasons then if I want to perform

INCREMENTAL backup then all blocks changed since yesterday will be backedup

if I want to perform INCREMENTAL CUMULATIVE backup then I am able as well

if instead of 3 I use 2 I will be able to perform INCREMENTAL backup since 2 is bigger or equal to 2

BUT I wont be able to perform INCREMENTAL CUMULATIVE backup since 2 is NOT bigger than 2


It depends what you want to do, incremental or incremental cumulative, there are two types of incremental backups in RMAN

To be honest, never understood the significance of this mapping...I mean why a "hourly" for Level 4 and not any other level?
Now, we are talking about incremental backups here.

Level 4 has to go for hourly backup because my daily backup is level 3 and if I use level 2 instead of 4 for my hourly backup, is 2 bigger or equal than 3? No for obvious reasons so I cannot backup changed blocks since last level 3 backup (therefore we cannot call it hourly can we?), I would have to find the last backup which has the level smaller or equal to my current backup level