The "buy three get the fourth free," i.e., 3+1, can often make buying wine at duty free a reasonable deal. The problem is that if you are buying and traveling alone, the four bottles are more than most countries' customs laws will allow you to bring in for free.
I have friends who have not declared the four bottles and been caught and paid fines that made the 3+1 a very expensive deal. Others' have declared the four bottles and been allowed to pass without paying extra. It's a calculated risk. I haven't bought wine at duty free in ages, but in my limited browsing experience, unless you avail yourself of the 3+1 deal, there are few bargains.