When exactly is season 4 starting?

As I've previously stated, it should start either at the end of this month, or mid-late February.

Don't wait for season 4 to play ranked. Games you win now will raise your mmr and secure you a higher placement for season 4!

soon (tm)

and with that being said, what Ozbirta is saying is your placement matches for S4 will draw from where you stood in ranked for the pre-season, so if you want to be placed higher and better in your S4 standings right off the bat start at it now!