Motion 4 was bundled with Final Cut Studio 3 (Final Cut Pro 7). There are a few copies left around with a pricetag hovering around $1500 (check Amazon.) There never was a way to download Motion 4 that I know of... legally.
With the exception of a few arcane filter image units, you can do everything you could do in Motion 4 in Motion 5, with the further exception of "round-tripping" in Final Cut Pro 7. It's not a terrible loss since you can export whatever media you need in FCP7 as ProRes 4444 out of Motion 5 and you won't be able to tell the difference other than it will be much faster to render the media as ProRes than it is to render as an embedded Motion Project.
You could try calling the Apple Store (1-800-MY-APPLE) and try pleading your case...
Good luck!