Good news, everyone...
Just saw it on the FM4 forums:
"The "Unicorn Hunter" cheevo will pop as soon as you import your profile from FM3, if you have any Unicorns in your FM3 garage."
So if you preorderd FM4 and tokk part in the Preorder Incentives program or got one of the 3 unicorns in FM3 you'll don't have to worry about this one! What a relief!!
Now I've just this comment made by red_hammer (thanks dude) in the achievement list here at x360a.
My doubt is:According to a post in forzacentral.com the 2007 Lamborghini Superleggera and the 2007 Ferrari 430 Scuderia are unicorn cars which you get via FM3 Profile Import if in FM3 Garage ~ Or buy in Auction House. Now both of this cars are available at the VIP Car Pack.
So... these cars could give you the achievement also?