Here's my write up of Book 4, and it's chock full of spoilers.
Black Tears
Written by L. Sprague de Camp & Lin Carter. Conan and his band of thieves escape an ambush by Turanian soldiers and pursue Vardanes, the man who betrayed them, into a desert waste. The superstitious fears of Conan's marauders get the better of them and they drug his wine and abandon him. He decides to press forward, but the harsh environment nearly kills him. He wakes in a lavishly furnished tent and his saviors tell him the tale of Akhlat the Accursed- a sorcerer summoned vampire gorgon that's been draining the life from the land and most of its inhabitants. The monster's power resides in it's third eye, which Conan cleaves in two. Black fluid pours from the wound like tears and she crumbles to dust.
Shadows in Zamboula
Written by Robert E. Howard. Conan rents a room at Aram Baksh's tavern where guests often end up in the local cannibals' roasting pit on the outskirts of town. After hacking his way off the menu, Conan rescues an intended side dish, a naked woman of course, and gets dragged into her drama involving her recently turned lunatic lover, and a conniving priest named Totrasmek. Sex being his eventual reward, he agrees to strong arm the priest for a cure for her boyfriend's case of the crazies. He meets Totrasmek's henchman, Baal-pteor, a strangler of Yota-pong, who laughs at him, which is a big mistake since it sends a "red wave of murder-lust driving across his vision" and they settle their differences with a strangling contest. After sending him to "Hell with his neck broken", Conan makes quick work of his boss who's been preoccupied watching the girl nearly dance herself to death avoiding imaginary cobras. She reneges on her promise of sex, but Conan has the last laugh as he has already stolen a valuable magic ring from her cured, but passed out lover. Even though he doesn't get the girl, he settles his score with the tavern owner before leaving Zamboula by feeding him to the cannibals, so I would still call that a happy ending.
The Devil In Iron
Written by Robert E. Howard. Conan reunites with his former gang, the kozaki, first introduced in the story Shadows In The Moonlight from Book 3, who have pulled themselves back together after King Yezdigerd's army made mince meat of them. Less than thrilled his lands are being ransacked by Conan again, another trap is set and unsurprisingly, a beautiful woman is the bait. Plans go sideways however due to the resurrection of an ancient being called Khosatral Khel on the island of Xapur where Conan is to be trapped. Though in human form, the statuesque creature is iron hard, has a voice like a bell, and compulsively smashes puny trespassers into a bloody pulp. Conan liberates a mystical blade from an enormous serpent watchdog, then channels his inner psycho killer and chases Khosatral Khel down and knifes him repeatedly, reducing him to his original form, which is so grotesque it can't even be described.
The Flame Knife
Written by Robert E. Howard & L. Sprague de Camp. The longest story in this collection, The Flame Knife pits Conan against enemies and allies alike, a man-faced yeti, and flesh eating ghouls. A slave girl Conan helps escape is later abducted by a cult called The Hidden Ones and taken to the secret city Yanaidar. Conan finds his way into the city and crosses paths with Olgerd Vladislav, the man who rescued him from crucifixion in the story A Witch Shall Be Born. Since Conan repaid him by stealing command of his men, Olgerd looks forward to settling the score. The story ends with an orgy of frenzied killing as three different factions war against each other, and the original inhabitants of the city emerge to feast on the survivors. I especially enjoyed the following passage that describes the mayhem at it's absolute peak.
"Conan did not waste breath trying to command order out of chaos. Craft and strategy had gone by the board; the fight would be decided by sheer muscle and ferocity. Hemmed in by howling madmen, there was nothing for him to do but split as many heads and spill as many guts as he could and let the gods of chance decide the issue."