It is unclear if by new cast he means just humans or also the Transformer characters (and thus the voice actors). The "new direction" means the "history" of the previous films will remain intact and possibly inform on events in the fourth movie. As I have said before, there is little reason there could not have been more than one NEST team defeating Decepticon threats not lead by Megatron. The budget cut seem significant but I wouldn't be surprised if half of that is recouped by hiring the new cheaper cast. The other half can be saved with careful use of special effects. Considering how Bay's on location shoots tend to be large in scale, the cost of space green screen sequences vs. on location shooting is about a wash in budget savings so that will be dictated more by story needs. Bay and writer Ehren Krueger still have at least six or so months to hammer out the first draft needed to get pre-production going so any or all these ideas could change. Below are the relevant segments, the full article can be found here.
“It’s kind of daunting and scary — you want to try to keep it going, to match what’s come before,” Bay said with a rare flash of self-doubt, or something close to it. ”We’ve accomplished a lot [with the first three films] but that doesn’t mean you get anything handed to you or that you’ve got everything figured out.”
“It’s a new cast,” Bay said with a matter-of-fact tone. “We’ve moving on to something different.”
“It’s not a reboot, that’s maybe the wrong word,” Bay said. “I don’t want to say reboot because then people will think we’re doing a Spider-Man and starting from the beginning. We’re not. We’re taking the story that you’ve seen — the story we’ve told in three movies already — and we’re taking it in a new direction. But we’re leaving those three as the history. It all still counts. I met with the writer before I went off to do ‘Pain and Gain’ and we talked about a bunch of ideas. We let that simmer for a bit. He’s been thinking about stuff and now we’re getting back together next week to see what we’ve got and to see if it gels.”
Could the new story involve a departure from Earth? “I think so, yeah, a little,” Bay said. “That feels like the way to go, doesn’t it? I want to go a little off but I don’t want to go too sci-fi. I still want to keep it grounded.That’s what works in these movies, that’s what makes it accessible.”
“[The budget's] going to be less, actually,” Bay said. “Our mandate is to cut about $30 million.”
“Here’s the thing, it’s tough to find someone who’s done these kind of movies and to have the complication of creating the new stuff that needs to be in this movie – not just characters but a new type of action, I hope – and that’s a lot for someone new to bite off. And so after this one I will leave it in the best hands possible. That’s the plan.”