It means the WBO featherweight title is vacant after the unbeaten American failed to make weight.
The 25-year-old Garcia knocked Lopez down with a right cross in the second round and put his opponent on the canvas again with a strong left-hand 1:34 into the fourth round.
"The disappointment was there because I lost my title," Garcia said after his 32nd consecutive victory.
"I was crying a little bit because I knew I wasn't the champion. But everybody got a chance to see I'm a good fighter. This was a satisfying victory."
"It's not that I wanted to make a statement. We just had a really good night," the Californian added after his 27th career knockout.
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