Does anyone have any idea at all if it is possible to take a bunch of vinyls and designs I made way back in Forza 4 and put them into Horizon 3 without having to individually recreate each of them which would take ages?
Cheers 
short answer no.
You can only import from current gen games on XB1 (nothing from X360)
[AB]
no
You can play forza horizon 1 one the xbox one .
And in forza horizon 1 you could import all the fm designs and vinyls.
I dont know if you can import fh 1 designs or vinyls to fh 3 if so thn that would be the pathway.
All the old stuff you should move from youre hd to the cloud on youre 360 x box first.