The Welsh striker made it 10 for the season while Frank Lampard scored the third in the 3-0 win. Hartson's brace follows the hat-trick he scored against Huddersfield in the second round.
"When I bought John people thought I'd paid a lot of money for him," Redknapp said. "But I always thought he was a great buy and I have been proved right.
"He's worth a lot more than I paid for him, no doubt about that. He's big, strong and powerful and he's only going to get better. I know I wouldn't want to play against him because he's such a handful.
"He's got an excellent scoring record and he's only 22, but he's only a baby still. He'll really be a big boy when he grows up."
Redknapp was delighted with the way his side put a full-strength Villa to the sword while Brian Little was unhappy at the way his team had performed. "We've had too many games like that this season," the Villa manager said.
COCA-COLA CUP Fourth-round draw: Leeds v Reading; Middlesbrough v Bolton; Chelsea v Southampton; Derby v Newcastle; West Ham v Walsall; Liverpool v Grimsby; Arsenal v Coventry City; Oxford United v Ipswich Town. Ties to played week beginning 17 November.