Headed into Apple's (NASDAQ:AAPL) fourth fiscal quarter of 2015, investors had questions about the company's growth potential beyond 2015. But with management seeming confident there is more growth to come, these concerns were soothed. Here are some key quotes from Apple management during the company's fourth-quarter earnings call explaining its growth opportunities, along with some other key insights.

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iPhone 6s. Image source: Apple.

Enterprise is a key catalyst
During Apple's fiscal year 2015, the company's total revenue increased by $51 billion compared to 2014, or 28%. But one large segment of Apple's business grew at an even faster pace. And no, this is not a direct reference to Apple's iPhone business (though a healthy iPhone upgrade cycle did benefit this segment). I'm talking about Apple's enterprise sales.

Fueled by Apple's strategic partnerships with IBM and Cisco and a strong product lineup, Apple estimated its enterprise markets accounted for $25 billion in annual Apple revenue during the last 12 months, representing impressive 40% growth over the prior year, Apple CFO Luca Maestri noted during the call. Even more, Cook said he expects more robust growth, calling the opportunity "a major growth vector for the future."

One particularly interesting note from Maestri on Apple's enterprise business was related to the value IBM specifically says it is creating with its mass transition to Macs.

IBM tells us that each Mac is saving $270 compared to a traditional PC, thanks to the much reduced support cost and better residual value. This is a terrific example of the kind of opportunity our devices offer to improve user experience and create value in the enterprise world.

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IBM CEO Virginia Rometty and Apple CEO Tim Cook. Image source: IBM

Cook advised investors during the call not to take the company's enterprise segment lightly.

[T]he enterprise business is not to be underestimated. I doubt very many people knew that we had $25 billion enterprise business that we quietly built in not too many years, but our penetration is low, but we have significant actions going on to really deepen that.

IBM has over 30,000 Macs within its company so far and is deploying 1,900 new Macs every week.

Apple is crushing it in China
China continues to be a key driver for Apple. The company's Greater China segment, which includes China, Hong Kong, and Taiwan, saw revenue increase 99%, year over year, during the company's fiscal fourth quarter. The segment, with $12.5 billion in revenue during the quarter, is now the company's second-largest segment (behind Apple's Americas segment) and well ahead of Europe revenue, which was $10.6 billion during the quarter.

At 24% of Apple's total revenue now, compared to just 14% in the year-ago quarter, the segment is becoming increasingly critical to the tech giant's overall results.

China, specifically, has been a major driver of the growth in Apple's Greater China segment.

"Total iPhone sales were up 120% in Mainland China [in Q4 compared to the year-ago quarter]," Maestri said. This was well ahead of Apple's overall iPhone growth for the quarter of 22%.

Apple Watch sales are growing
Apple Watch is still a small business for Apple, but it's looking good, Cook said.

"Sales of Apple Watch were also up sequentially and were ahead of our expectations," he noted.

The company attributed its 61% year-over-year growth in its "other products" category to Apple Watch.Apple Watch Tmf

Extrapolating from the company's sequential growth in "other products," Apple Watch sales likely grew by about $407 million sequentially -- not bad for an entirely new product category still in its first year.

Android switchers are at an all-time high
Some investors were worried the tech giant's year-ago comparisons would be tough to beat. But Apple surprised analysts by guiding for 8% to 11% year-over-year revenue growth in constant currency during the company's holiday quarter, or its fiscal first quarter of 2015. Investors can thank Android switchers for this optimistic outlook.

"We believe that iPhone will grow in Q1 and we base that on what we are seeing from a switcher point of view; we recorded the highest rate on record for Android switches last quarter, at 30%," Cook said. Explaining the metric, Cook noted the 30% figure cites the percentage of iPhone buyers that were upgrading from an Android device.

Cook also cited a low upgrade rate from its install base prior to iPhone 6 and iPhone 6 Plus, China, emerging markets, Apple TV, the App Store, and Apple Watch as expected catalysts for more growth.

Management was very upbeat during the quarter -- and for good reason: The company is firing on all cylinders. Going forward, investors should continue to check in on management's sentiments about its business. As the tech giant grows larger and larger, revenue growth will be more difficult to achieve, and expert capital allocation will subsequently become increasingly important. Investors should continue to monitor how the company manages its growth prospects on both an absolute and per-share basis.

Daniel Sparks owns shares of Apple. The Motley Fool owns shares of and recommends Alphabet (A and C shares) and Apple. The Motley Fool recommends Cisco Systems. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.