I agree to Quadro40k,
it is not that easy to become downloads and by all means it is quite a thing of luck to get it in FH4 due to the new presentation system.
I started designing in FH3 but only a few cars and because i got bored in FH4 i made much more there and now i love the editor. Try to make some good photos of your designs, upload them here and in some Forza Clubs on XBox Live. Try to get in contact with the community and you will get some downloads…and last but not least: don´t stress yourself to get the status by all means. In the first line you should make your designs for your own, do what YOU like and don´t try to create things you think others may download.
For example: i got totally surprised as i made a simple design for the Mitsubishi Eclipse. I made it and was so happy with the result, that i decided to share it to others. Guess what: nearly EVERYONE made the Paul Walker design from Fast & Furious, so there where like 20 times this design and my simple but different one. Some people started to download the alternative to the ever same F&F design and my design got a lot of attention because of that. By now it is one of the most downloaded in my Creative hub, even if i have some others i spend hours creating.
So basically you need a good portion of luck, some creative thoughts you can realize and of course much patience 