I want to buy a Phantom 3 Pro, but now I'm April it will be a year since the Phantom 3. Do you think the Phantom 4 will come out again in April, and should I wait for it. If it will come out around that time then I will wait.

Wondering the same...

There will always be new models and your guess is as good as mine to the timing of any new release.
That said, with the current pricing DJI have they are all pretty attractive, the Advanced is a great price point IMHO..
so, how long do you want to wait to have some fun and what will you be missing between now and then for opportunities to fly?

Good luck!