The hosts' David Miller had earlier struck a first one-day international century as he caressed 11 fours and three sixes in an unbeaten 130 from 133 balls to take the Proteas to 262 for eight in their 50 overs.
Russell took West Indies past their victory target with nine balls and just a single wicket remaining, hitting five sixes in his unbeaten 64 from 40 balls.
He put on 27 for the final wicket with Sheldon Cottrell (one) as the tourists pulled a game back in the series to go into the fifth and final match in Pretoria on Wednesday trailing 3-1.
The victory is a rare high-point on a disappointing tour for the Caribbean side, but should give them some confidence going into next month's World Cup, albeit against a much-changed Proteas lineup.
West Indies lost early wickets but rallied with a 93-run sixth wicket partnership between Marlon Samuels (68) and Darren Sammy (51), before Russell strode to the crease and proved the perfect finisher.
West Indies won the toss and sent South Africa into bat, with the Proteas rotating their squad and resting Hashim Amla, Dale Steyn, Vernon Philander and Imran Tahir.
They slipped to 76 for four, but left-hander Miller abandoned his usual power-hitting style to steady the innings and along with JP Duminy (43) put on 90 for the fifth wicket.
Some lusty blows in the final three overs took the innings score past 250 and gave Miller the record for the highest score by a South African number five in ODI cricket -- beating the previous best of 121 by Jonty Rhodes that has stood for 19 years.
The West Indies reply was not helped by the run outs of Leon Johnson (18), Jonathan Carter (five) and Carlos Brathwaite (11), but Russell kept them in the game as wickets fell around him.
The tourists will be grateful to the dismissal review system though after Russell was given out by Australian umpire Steve Davis on 40, caught behind off Wayne Parnell.
He sent it straight upstairs and with no sign of an edge, the dismissal that would likely have won the game for South Africa was overturned.
(Reporting by Nick Said in Cape Town; editing by Toby Davis)
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