By Duff McDonald, contributor

It used to be one of the favorite thought exercises in the technology realm — putting together a list of the Four Horsemen of Technology, the four most important public companies of their time.

In the 1990s, most people could agree on the same list. There was Microsoft, undisputed king. There was Intel, which had somehow transcended being a trade brand into one known by consumers. (Not that people ever understood just what Pentium meant. But that’s neither here nor there.) There was Oracle, the database favorite. And there was Dell, low-cost champion in the all-important personal computer category.

The dawning of the Internet and mobile eras saw that list swept clean. While there hasn’t been universal agreement on a precise list of four ever since, there were a handful of companies that made long-running appearances. Cisco took top spot for a time, before the communications equipment company’s heady growth days ended. Research in Motion was the be-all, end-all for the decade of the Blackberry. Amazon showed up as the great disrupter of the Internet era. And then came Google, the new promise of all things technology. Whereas Microsoft was evil, Google was good. Google was the future.

Then the tech and credit bubbles burst, and people stopped making their lists.

When I called Pip Coburn, CEO of Coburn Ventures, a technology advisory company, to talk about the four horsemen of today, his answer caught me off guard. “There’s only one horseman,” he said. “It’s Apple. There’s nobody else.”

On the face of it, he’s right. Apple is the most valuable technology company in existence with a market capitalization of $233 billion. It is also the only company that technology investors can agree on. “You have no idea how many conversations about investing I have that start in general and end up on Apple,” says Coburn. “And then they just stay there.”

Coburn points out the reason why the original four have had a tough time of late. We’re in our tenth year of doing more with less in the enterprise, and what power IT buyers used to have is now in the hands of CEOs and CFOs who are concerned with margins above all. If your sweet spot was the enterprise, it’s now a sour one. As a result, Microsoft, Intel, and Oracle—while all still large and meaningful—no longer dictate the routes of change in technology. Dell is another story. The low-cost competitor has turned into a high-frequency disaster story of late.

Horsemen of the future

The news is not so good for the later members either. Cisco is still a huge and well-run company, but they’ve lost their mojo. Witness the company’s effort to get everyone on videophones. Do you have one yet? And the biggest result of this week’s news that the iPhone might be available on Verizon next year was that Research in Motion’s stock fell 6.1%.

But what of Google? Could the search giant not be the Tonto to Apple’s Lone Ranger? It might have been, but when Eric Schmidt got himself kicked off Apple’s board last summer when Google started messing around in mobile phones, that partnership was obviously over. Not that Steve Jobs needs a partner, but he was clearly considering bringing Google along for the ride. No longer.

Of course, the point of a list of four, you might say, is to make a list of four, no matter what the allure of putting just one company on. Who would Coburn consider adding to the list if he had a gun to his head?

Amazon, for one. But not for the reason you would think. Yes, he says, it is a great retailer. But its biggest impact in the next five years might be in the realm of cloud computing, where it has nothing to lose by going for broke to lure in corporate customers. Coburn also likes Netflix for the combination of corporate culture and singular focus.

Facebook is a contender, if one assumes that they finally decide to go public. Although at the rate social media is moving these days, Facebook’s management team is going to have to prove themselves a lot more nimble than they already have if they want to avoid becoming the next MySpace, run over by the next big thing.

Here’s Coburn’s surprise entry, though: Microsoft. Remember when we all hated Microsoft? Well, we don’t any longer. And while the company has failed to commit to the mobile realm with the conviction it has brought to other new businesses in recent years—think security, Internet, and gaming—that’s changed. It finally appears to be taking mobile as seriously as, well, Apple.