Is it safe to take an Ambien and then drink a beer?

Yes, it's perfectly safe unless you still feel the beer in your system more than just a little. there's a large degree of synergy between the two drugs, but it only really manifests when they're coadministrated together.

it's basically only dangerous if you take them too close to eachother, almost all the alcohol in your body will have been excreted after 4 hours and what little is left will only give a slight boost to the ambien. i might recommend taking 2/3s of your dose if you had drank a lot.