It's completely impossible to shop for a 4-socket non-magic armour.
If you mean a magic armour with the Jeweler's affix, that'd require alvl 55. The earliest armour you can get 4 sockets on is Gothic. You'd need clvl 67+ and shop at Hralti (Nightmare), Halbu (Nightmare), Larzuk (Nightmare), Charsi (Hell) or Gheed (Hell). If you're clvl 72+, any body armour has the possibility of getting the jeweler's affix (though not all armours can have 4 sockets).
If you mean a magic armour with the Jeweler's affix, that'd require alvl 55. The earliest armour you can get 4 sockets on is Gothic. You'd need clvl 67+ and shop at Hralti (Nightmare), Halbu (Nightmare), Larzuk (Nightmare), Charsi (Hell) or Gheed (Hell). If you're clvl 72+, any body armour has the possibility of getting the jeweler's affix (though not all armours can have 4 sockets).