The ABC Family press release about the casting news clarifies that Tyler "will still be seen in the upcoming summer season of Pretty Little Liars." That makes sense. The Season 4 summer season has already been written, and is in the middle of filming, while Ravenswood is still in the midst of casting. There's no conflict there.
However, the phrasing in the press release also implies that Tyler will not "still be seen" on Pretty Little Liars after the summer season — that is, we should expect to stop seeing him in Rosewood during the Season 4 winter season.
Another clue that Tyler is gone for good is his castmates’ responses to the news. Ashley Benson (Hanna), who has the most scenes with Tyler as his on-screen girlfriend, said “I'm going to miss my little love bug @tylerjblackburn. Sooooo excited for ravenswood xxxxxxxx.” This doesn’t sound like the kind of tweet you send when someone is off filming a guest spot on the next soundstage over.
Plus, there is also an element of common sense. Tyler is billed in the press release as "headlining" the spin-off, which doesn't leave a lot of time for appearing on other shows at the same time.
Cheer up, PLLers! Ravenswood is not so far away from Rosewood. Maybe Caleb can at least visit for an episode or two. And either way, we still get to have Tyler gracing our screens every week, in a big way. The other way characters tend to exit PLL is in a body bag. We’ll take a relocation to Ravenswood over that any day!