If I started on Steam, do I need to buy 4.0 on Steam?

I've moved to the PS4 and was wondering.

You should be fine with just the PS4 version of 4.0, unless you intend to play it on a PC as well.

If you ever want to move back to PC, unless they announce otherwise, you will need to buy the expansion through Steam.

Thanks, I remember hearing about some people having problems with Steam when Heavensward launched, but that might have just been with preorders.

The preorder issues were basically because we could buy the non-Steam PC version a long time (nearly a month?) before it became available on Steam. But it still became available on Steam before launch (which is what SE had said would happen) so in the end it didn't make any difference, it was just a lot of worried people who wanted to lock in the preorder asap.