Google Inc. nurtured YouTube into a cultural phenomenon, attracting more than one billion users each month. Still, YouTube hasn’t become a profitable business.

The online-video unit posted revenue of about $4 billion in 2014, up from $3 billion a year earlier, according to two people familiar with its financials, as advertiser-friendly moves enticed some big brands to spend more. But while YouTube accounted for about 6% of Google’s overall sales last year, it didn’t contribute to earnings. After paying for content, and the...