In the bottom of the first inning, Jose Altuve hit a fly ball towards the right center fence for a possible two-run home run to tie the game with Boston.
Red Sox outfielder Mookie Betts leaped toward the wall with his glove extended toward fans awaiting a possible souvenir.
The ball went out of play, but right field umpire Joe West called fan interference which effectively meant Altuve was out. Under fan interference rules, an out is called if a fielder's catch that would have been an out was affected by a fan in the stands.
Here is the full definition of Spectator Interference, according to the MLB.
In every case of spectator interference with a batted or thrown ball, the ball shall be declared dead and the baserunners can be placed where the umpire determines they would have been without the interference. When a spectator clearly prevents a fielder from catching a fly ball by reaching onto the field of play, the batter shall be ruled out. But no interference is called if a spectator comes in contact with a batted or thrown ball without reaching onto the field of play -- even if a fielder might have caught the ball had the spectator not been there.
WATCH: Was this fan interference? You decide
An automatic replay review did not have enough evidence to overturn the initial call, making the out stand.
Eyewitness News reporter Nick Natario spoke with fan Troy Caldwell, who is at the center of the controversial call. He was ruled to have interfered with the play.
"Troy, how are you feeling right now? Did you touch the glove?" Nick asked as Caldwell returned to his seat after the incident.
HOME RUN CONTROVERSY: Was it a home run or fan interference? I just went and asked the #Astros fan himself. Troy said without a doubt it should’ve been a home run. #NeverSettle https://t.co/KguuJO5AFf pic.twitter.com/CvVMOKnKLX— Nick Natario (@NickABC13) October 18, 2018
According to Caldwell, the hit was "without a doubt" a home run.
Caldwell later answered Nick's question when he told ESPN that he never touched Betts' glove and it was another fan named Jared who did.
"I never touched his glove, I can guarantee you that. But I definitely touched the ball," Caldwell claims.
After a back-and-forth game, the Astros wound up losing Game 4 by two runs. Natario caught up with the fan as he exited Minute Maid Park to get his thoughts:
"If Boston wins this series, then Joe West is the MVP. I'll give that to you right now," Caldwell added when talking to ESPN.
The World Series defending champion Astros face elimination Thursday night, now that they trail the Red Sox in the series 3-1.
Justin Verlander will get the start. First pitch for game 5 is at 7 p.m.
SEE MORE: Women behind the Astros weigh in on home run controversy
SEE MORE: ALCS Game 4 live scoring log