Are they "pulling a PR tactic" or are they just projects that don't work out? Not every project somebody is Excited To Announce gets finished or released. There's no reason to think they invent them to fake success. Star Trek 4 was assumed to be a going concern when it was first announced. Franchise was going strong, writer keen, story idea already drafted. That things shuddered to a halt later on doesn't make that initial enthusiasm anything other than genuine.
It wasn't going as strong as hoped, especially to foreign investors. The returns of the films were moderate. They didn't have super strong ROI and Paramount relied on some foreign investments for ST Beyond. Announcing the 4th film during the week of it's release with Hemsworth listed as cast was a move to convince investors that all was good with the franchise, and it was worth continuing. That tactic doesn't necessarily mean the film will actually happen, as we've seen with the Kelvin house of cards falling, and investors backing out.