What will happen if i mix 8mg of zanaflex and 4mg of xanax (alprazolam) as a sleep aid?

Dangerous sedation: the mixture of a muscle relaxer ( Zanaflex) and a tranquilizer ( benzodiazepine ) especially a 4 mg dose can lead to severe sedation and loss of consciousness: why are you taking this combination. I suggest ER evaluation- NOW!

You could get sick: and potentially even die. I doubt from your question that either of these are prescribed, so I am guessing you are getting them illegally. 4 mg of Xanax (alprazolam) is a HUGE dose, equivalent to 40-80 mg of valium. It is usually given in 0.25- 0.5 mg doses in people first getting it, and certainly no more than 1 mg. Neither of these drugs are approved for insomnia. Talk to your doctor or health provider.