Which four star hero do I choose?

Heracles. Emiya isn't gonna be good for a while but Heracles is an immortal back-liner with his bond CE.

It's a matter of who you currently have:

No single target berserker/backline? Obviously Heracles. Can do his job flawlessly especially at max bond.

Need a good crit star generator and crit machine? Emiya. When he gets his strengthening he becomes one of the best SR archers out there. One of his most notable skills will be Hawkeye which increases crit star gen and crit dmg for 3 turns. Both at an absurd 100%.