In what could prove to be the Ukrainian's last-ever bout before embarking on a career in politics, a cut over Char's right eye brought proceedings to an end with 56 seconds remaining in the fourth round.
"I was a bit disappointed as I wanted to win today by a real knockout, not a technical one," said Klitschko afterwards.
"I was ready to continue the bout and Char was also ready for further boxing but it was the decision of the doctor and the referee, who are responsible for it and we could do nothing about it.
Fighting in the 30,000-seater Olimpiyski arena, the bout was a one-sided contest from the off, with Klitschko stalking the ring behind his jab and landing a couple of early shorts.
He put the former kickboxer down in the second round, landing a left hook which saw Char counted to eight. And, when a similar shot wounded Char in the fourth, the referee stepped in.
Char was disgruntled with the call, kicking the ropes, but the amount of blood he was shedding meant it was impossible for him to carry on.
"In the first two rounds Char was concentrated on defence only and it was slightly difficult for me as I like it better when my opponents are fighting back," Klitschko said.
"In the third and fourth he started to come out of his shell more and I had more chances to attack.
"But Char is a true fighter. He was ready to fight to the end. I think this defeat will not stop him and I believe he has chances for a promising career in boxing."
Meanwhile Andre Ward knocked down Chad Dawson three times before winning on a technical knockout in the 10th round to defend his WBC-WBA super middleweight titles in front of a hometown Californian crowd.
Ward, 26-0 going into the fight, knocked down Dawson late in the third round and early in the fourth to take control of the fight, before finishing him off half way through the tenth round.
Dawson, the WBC light heavyweight champion, had moved down a weight class and took the fight in Ward's home town of Oakland.