Joey Porter Jr. was actually traded from the Steelers to the Dallas Cowboys on October 1, just days before Week 4! So your hunch about him and the Cowboys is correct.
Here's the straight answer: he has "a chance" to play Week 4 against the Texans, but it's not locked in. His conditioning concerns that sidelined him in Pittsburgh have reportedly cleared, and he pulled an all-nighter after the trade, arriving in Dallas in time to practice with the team on Thursday. The Cowboys are desperate for secondary help — they ranked among the NFL's worst in coverage DVOA heading into Week 4.
(from the Joey Porter Jr. page)
Expert take: On The Fantasy Headliners, the hosts noted earlier this week that the Cowboys' secondary didn't know if they'd be getting Porter back yet — but that was pre-trade. Now that he's in Dallas and practicing, the tone is shifting toward optimism for a Week 4 debut.
Bottom line: Don't count on him for a full workload in his first game with the team, but he's trending toward active for Week 4.