Wembanyama, back after missing game three because of a concussion he sustained in game two, scored 27 points with 11 rebounds, three assists, four steals and seven blocked shots.
"I had lots of emotions in me before the game, obviously," said the 22-year-old French superstar, already named the 2026 NBA Defensive Player of the Year and in the running for Most Valuable Player.
"Excitement, frustration, so I let it all out tonight," said Wembanyama, who said the Spurs were "amazing" after his injury.
But he was "unhappy" with how the protocol - which requires a gradual return to basketball activity monitored by medical staff - was handled by other parties involved.
"I've been really healthy, starting on day one after the injury," he said.
Fox led all scorers with 28 points, and the Spurs out-scored the Trail Blazers 73-35 in the second half to move within one win of advancing to the Western Conference semi-finals.
The Spurs, seeded second in the West, can clinch the best-of-seven series when they host game five on Tuesday.
"When we got in that hole, we just had to figure out a way to play the right way," Fox said. "Not necessarily play better. If you don't make shots, it is what it is ... but the way that we got down was, we felt like, all our fault. Third quarter we played the right way.
"We kept them out of the paint and that's why we were able to get back in this game."
Deni Avdija scored 26 points and Jrue Holiday added 20 for the Trail Blazers, but they were the only Portland starters who scored in the second half.
Celtics rout Sixers
The Boston Celtics crushed the 76ers 128-96 in Philadelphia to take a 3-1 lead in their Eastern Conference series and a chance to close it out at home on Tuesday.
Payton Pritchard scored 32 points off the bench and Jayson Tatum scored 30 points with seven rebounds and 11 assists for the Celtics, who led by 16 points after one quarter and by double digits the rest of the way.
Joel Embiid led the 76ers with 26 points and 10 rebounds in his first game since an emergency appendectomy on April 9. Tyrese Maxey added 22 points but the 76ers couldn't counter the Celtics' rain of 24 three-pointers.