I don't care about the account I just want the money back
If you don't care about the rest of $4000 then something is not good like the other poster said. You know what you did otherwise you would fight for the whole amount.
So to make this short , I had a chat with my partner and he made an account, previously on a similar address
I am not a Rollbit fan but honestly speaking this was your mistake and you can not blame Rollbit for that. They are following the general terms.