"I threw all my pitches, threw a lot of strikes, got up and down and got to my pitch count," Cosart said. "I'm ready to go."
Having thrown 65 pitches, Cosart wouldn't have been available for the Padres until Wednesday or Thursday.
It's unclear who will head to the Minors to clear room for Cosart, but it's unlikely the Padres would carry nine relievers.
In all likelihood, the Padres will use Cosart out of the bullpen, as an option for long relief or a spot start, should the need arise. Sunday marked Cosart's second rehab outing, after he pitched two innings at Class A Advanced Lake Elsinore last week.
"The reason we sent him back out for four [innings] was that if we needed a spot start if something happened, he's a prime candidate for that," said Padres manager Andy Green. "Also, now, if you need someone in the 'pen for length, he's capable of giving us five innings, six innings, because he gave us four yesterday. We feel good about where he is, and he's pretty pumped about how he threw the ball."
In three appearances this season, Cosart has a 3.24 ERA. He struggled in his lone relief outing in Los Angeles, allowing two runs on a hit and three walks.
The bullpen is still new to Cosart, who had appeared in relief only once before this season.
"It's still going to be a learning process, but I feel more comfortable, more prepared for that role," Cosart said. "It goes back to being ready when your name's called. When the phone rings at any time, it could be you."
Cosart added that pitching out of the 'pen has given him a new appreciation for the job done by relievers.
"It's not easy," he said. "It's really hard. There's the uncertainty of whether you're going to pitch or not. It's a different mindset, coming out of the bullpen."