To add more to this, not essentially 4K is the only thing behind L1, even HD streams can be with L1.
It's entirely the services' choice to use what they want, like they can even put SD stream behind L1 and leave 4K for L3 (this happens widely in lesser known services & L2 is hardly used). Also Amazon's 4K is different from Netflix's 4K considering the key revocation TAT. So everything changes from service to service.