This is actually quite disappointing. Not that they are discontinuing their efforts with Daydream VR, but that they made the decision to specifically prevent Pixel 4 from using it. For a moment I considered that possibly the 90Hz display might be a complication with it, but the phone could easily be forced into 60Hz when VR mode were activated.
Active prevention of using a prior product with one which would be compatible if not for that tactic is a pretty petty move on Google's part - and in my case, isn't tipping my decision (which is pretty close on the line between keeping my 3 XL and going to a 4 XL) the way Google would want.
I'm not a big Daydream user, but I do still have mine and use it when on long flights.