The Australian broke his hand at the Dutch Grand Prix in August and missed five races before returning in Austin, Texas, last weekend, where he finished 15th.
"Already Sunday night in Austin I was just wanting it to be Friday practice, I wanted to get back in the car and it felt like there were some things left on the table," said Ricciardo, who starts ahead of Red Bull's Sergio Perez - the driver he hopes eventually to replace. "There were some things also with setup that we knew we could try this weekend, having a more conventional weekend."
The United States Grand Prix in Austin ran a sprint format with just one practice session and a standalone 100km race on the Saturday.
"From lap one yesterday (in practice) I felt really confident. P4 is maybe a little more than we thought," Ricciardo said. "Coming into quali it wasn't about getting into Q3, it was how far can we get inside. As a team we definitely had confidence."
Ricciardo has made only three race starts since he launched his comeback after leaving McLaren last year by mutual agreement.
He said holding on to fourth in the race would be "awesome and crazy, but I don’t know if I should use the word ‘crazy’, because I think today wasn’t a fluke."
"I think there was definitely a positive chip on my shoulder as well," he added. "I know I haven't done much this year race-wise, but I feel a lot like my old self and I feel like I can do well."
He said the hand was mended, although he was "not ready for a boxing ring yet".
Ricciardo's Japanese team-mate Yuki Tsunoda, who provided useful aerodynamic tows in the first two phases of Saturday's qualifying, will start the race from the back at the Autodromo Hermanos Rodriguez after power unit penalties.
