ITV managed to source a widescreen version from Warner Bros because they broadcast this film earlier tonight on ITV4 and it appeared to be true widescreen and not a stretched 4:3 image.

http://www.dvdverdict.com/reviews/policeacademy5.php
They seem to confirm that (at least 5) was matted for widescreen. (or that WB lacked a "widescreen" copy Warner Bros doesn't seem to be the type to just lose or misplace films, so I doubt even these movies got junked and thrown out)
Excerpt:
The official reason: "widescreen elements were unable to be located at the time." Well, the Police Academy films were shot in open matte, which means that the top and bottom of the image in a full frame transfer reveals what the director didn't want you to see. Director Alan Meyerson is still alive. Why not consult him and simply matte the image to recreate the 1.85:1 aspect ratio? After all, Warner Bros. matted Kung Fu when they shouldn't have. Anyway, the full frame image is grainy and drained of color. It looks like bad VHS rather than the crisp clarity of our favored medium.
This means, if that excerpt is true, that they used cheaper film stock to film these movies in, framing them for widescreen to later be matted out.

Most of the films, except for the direct to video ones, were shot in 4x3 BUT were composed for 1.85:1 for theatrical presentation thus the DVD's aren't presented correctly at 4x3.
"Don't flake on me, man, they're vampires, send em' to hell."

Rafe Guttman

I can't speak for 5 (though probably the same deal as 4) but with 4, it was a matted widescreen. This is why the DVD's are always fullscreen, it was shot that way. If you have a zoom feature on your system, Police Academy 4 is not ruined by zooming in (no heads cut off, nothing important lost), which is how a widescreen or the theatrical version would look anyway And you're right, unlike part 4, part 1 fullscreen DVD is cropped, zooming in is no good for part 1.