Strategy to increase rents to market rent

[Mica Moore]

I am looking into buying a 4-plex that has existing tenants in 3 units ( I plan to owner occupy unit 4). All of the occupied units are at $600 per month, but market rent should be $800 to $850.

What strategy can be used to get rents up to market rent?

i don't want to do a whole $200/$250 rent increase at once and then end up with 3 vacant units.

Do I just raise it $50 a month each year? Feels like that would take 4 or more years to get it to market rent though ( which is better than a vacant building). 

Should I be upfront with the tenants and tell them that the units are currently below market rent, and therefore, to expect annual increases? 

Not quite sure how to go about this.