She'll be travelling alongside K9, sandwiched between the events of Seasons 15 and 16. She is/will be voiced by Jane Slavin, who has already voiced a shit ton of characters in Tom's audios, most notably as the voice of the Laan in THE SANDS OF LIFE, so... yeah, Four's now travelling with a Laan. CANON.
Obviously all you canon snobs will immediately start throthing at the mouths because "DURR, IF SHE'S SUPPOSED TO BE CANON, THEN WHY DOESN'T FOUR REMEMBER HER ALONGSIDE HIS OTHER COMPANIONS IN THE FINAL MOMENTS OF LOGOPOLIS?!". Wellllll screw you; Fivey doesn't remember Leela when wired into that duplicate machine in Resurrection of the Daleks, so does that technically mean Leela's not canon outside of her run? Exactly.