Did you leave the download window popup alone until the download completed? FM4 DLC downloads through that popup window only, it won’t go to the download queue like most downloads would. If you close that download window, the download stops completely.
I always bring this up anytime someone has issues with FM4 DLC because, as simple as it is, it’s usually the problem.
I had a similar problem that was more user error than anything. Do you use an external memory device to save your games to? If so, if you load the game through the external device, the Porsche pack downloads to the console memory. So you’ll have to move it from the console memory to the external memory. Just a thought.