if you mean alprazolam(xanax) the highest dose of it in a single pill is 2mg and it is a benzodiazepine and is an anti anxiety med that's as far as im going into detail..
do some research on it, we aren't here to do all the work for you..
try google, drugs.com, rxlist.com wiki etc..
BUT it is not good for dogs to take a large amount as they could overdose, but they do prescribe certain benzos to dogs to calm them during long travel or before surgery.
Double check the drugs name unless there are 4mg extended release xanax i don't think the pill you're talking about exists, or i might have assumed the wrong drug with your misspelling.