Guys, after the beta I completely wiped my PS4 Pro (I wanted to do that after the 5.00 update at some point to start from fresh). After the wipe I only re-installed PCARS 2 and Horizon Zero Dawn for now, and then last Monday the pre-load of GT:Sport.
When I started GT:Sport yesterday I got a prompt asking if I wanted my beta stuff (cars, credits, mileage points) transferred to the game or not, accompanied with a warning that they can only be transferred once.
Since there was not a single bit of beta code left on my PS4 due to the wipe and I don’t have PSN+, the Sony GT servers must have saved my account credentials and kept my beta info ready on it. Since my PSN account is the only way they could link it I would conclude all your beta info is saved in your PSN account. If you still have the beta running on your PS4 or not doesn’t matter, make sure you use the same account as the one you played the beta with.