> 4. Invest your savings and live off of gains.
this option is not really like the other 3 listed, since it's a separate category of passive income and is non-exclusionary unlike the rest.
One should always be investing any excess income anyway, regardless of what path one chooses to follow. Unfortunately, the risk of having _only_ investment income to sustain you is that you cannot make risky investments, and so must have large amount of capital to produce less risky but more guaranteed returns. FIRE comes to mind when one goes down this course of action.