They said last night that because of the way licensing works with the expansion you WILL need to buy the expansion for both, HOWEVER if you are trying to get the Collector's Edition you only need to buy it for one of the platforms (IE: PC Collector's Edition, PS4 Standard Edition or vice-versa).

If you opt out to buying both day one and say you only buy the pc expansion, then if you log out in a Heavensward zone you will NOT be able to log into that character from the PS4. Hope this helps, do keep in mind too that the expansion is only 40 so its not marked at that 60 dollar price point like the original game was...so there is that.