Question: I am currently finishing my masters in Germany and I will start my PhD soon. I will be having a TV-L contract, however, I haven't acquired my official diploma yet. In case that the bureaucratic stuff are not finished before my starting day, would I still be able to sign the contract by proving I have submitted my thesis (or getting something like 4,0 bescheinigung)? Or do I have to wait until I am officially graduated?

Answer: 
I know some people who signed their contract with "4,0 Bescheinigung" and got the final mark afterwards. As far as I know submitting your thesis is not enough, you have to prove that you passed your masters.

But it would be better if you ask your supervisor or the people in the HR.
To sign a TV-L 13 contract, you definitly need a proof that you have a master's degree (or something equivalent). I think a 4.0-Bescheinigung should suffice but this might also depend on your university's policies for that matter. As pointed out already, go check with your university's HR department.