Can you mix your own hydroquinone like a 6-8%

Can you? Yes. Should you? Probably not.

I have a 4% how could I make it stronger I'm trying to fix my issue since I can't get a solution from my physician

That is a very high dosage. You may cause irreversible skin damage.

Why do you want to burn your face off? There is a very good reason this solution is sold at only 4% for your safety!