The 24-year-old, who is the top-ranked Asian-born man ever, tweeted, "Just saw the new rankings. Very proud of another step...#4 this week."
Nishikori, last year's US Open runner-up, now trails only Novak Djokovic, Roger Federer and Rafa Nadal after leapfrogging Britain's Andy Murray, meaning he will avoid the top three until the semi-finals at the major tournaments.
"He has the touch of a genius," said former Japan number one Shuzo Matsuoka. "He has the imagination and shots you just can't teach."
Nishikori recently won his third successive Memphis Open, his eighth ATP Tour title. He is fast closing in on Nadal and trails the 14-times grand slam champion by just 260 ranking points.
"It's an achievement that's almost impossible to explain," Matsuoka told Japan's Sankei Sports, comparing the skinny Nishikori to a sumo grand champion. "He's really playing 'yokozuna'-grade tennis."
Nishikori burst onto the scene as an 18-year-old by winning in Delray Beach as a 244th-ranked qualifier in 2008, and is a huge celebrity in Japan.
Nadal back in ATP top three
Former world number one Nadal climbed back into third place in the latest ATP Rankings thanks to his Argentine Open win.
In Buenos Aires on Sunday, the Spanish superstar ended a title drought that stretched all the way back to his ninth French Open title at Roland Garros last June.
Serbia's Djokovic remains in first place despite losing to second-ranked Switzerland's Roger Federer in the final of the Dubai Open on Saturday.
Like Sports on Facebook, follow @ETribuneSports on Twitter to stay informed and join in the conversation.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ