The fight was stopped with 56sec remaining in the round.
Klitschko improved his record to 45-2, while Charr, who was unbeaten before his meeting with Ukraine’s 41-year-old veteran, fell to 21-1.
Fighting in the 30,000-seater Olimpiyski indoor sports arena, the hugely-experienced Klitschko started the bout as a hot favourite in what was his 17th world heavyweight championship fight.
Charr, a 27-year-old Beirut-born former kickboxer, brought with him an unblemished record in 21 fights and a seventh ranking by the WBC, but none of his opponents had been of the highest calibre.
The much smaller Charr was immediately on the defensive as the towering Klitschko stalked him behind his powerful jab and it was only in the closing seconds that the German attempted a couple of tentative punches.
Klitschko stayed well on top in the second round and he sent Charr crashing to the floor seconds before the bell for a standing eight count with a left hook to the side of the head.
The German challenger recovered well though and caught Klitschko with a brief flurry of punches in the third, but it was the Ukrainian who again won the round.
The fight then came to an aprupt end in the fourth when a Klitschko left hook brought a gush of blood from above Charr’s right eye and the referee was quick to step in and end the contest.
The German was furious with the decision, kicking the ropes in his anger, but it was clear from the amount of blood that there was no way he could continue.