If it's your first time with Xanax 4mg is gonna put you on your ass, multiple times.
No high-killing. This is a straight up "the whole is greater than the sum of it's parts" combination. You know.... Synergy. Put the two together and you'll be exponentially more intoxicated than either of them on their own.
For real, though, if you're under 150lbs and have never fucked around with Xanax before, stick with 2mg at most. I'd honestly recommend .5mg at a time because that shit will creep up on you if you're constantly distracted, which you probably will be because Xanax.
Biggest drawback people tend to mention is that the benzo' kind of dulls the psychedelic aspect of weed. Personally, I'm cool with that because I smoke to relax. It's something to keep in mind, though.