I am sorry to say, I think you should go or have someone go in your behalf. A four year old cannot be trusted to fend for herself. Are there any other people there that you could trust to look out for her and speak to them ahead?? If you go could you drop her off and check in after a bit and if he is drinking remove her?? I don't know.
I have it in my divorce decree that my X is not allowed to drink at all around our children or have them in an environment where there is active drinking or alcohol being served. It's tight, but it is for this exact reason.
Hugs. Good Luck!