The tablet market is going to continue to grow and expand, but that growth will largely belong to Apple’s iPad, according to one mobile analysis firm.

Gartner issued a four-year analysis forecast of the tablet market today, expecting that while the tablet market will increase from 17.6 million total sales in 2010 to 326.3 million by 2015, most of those tablet sales will be iPads. In fact, Gartner expects about 146 million iPad sales in 2015.

It’s not exactly a new position that Gartner is taking – the iPad dominates the market today, and is the front-runner that all other tablets are running to catch up with – but the number of sales certainly is. Other analysts have noted that they expect tablets running Google’s Android platform to eventually overtake the iPad, as device makers start to differentiate, expand into lower- and higher-priced options, and generally find ways to compete that don’t involve aping Apple’s concepts. But Gartner sees Apple finding ways to remain at the front of the pack until midway through the next decade, at least.

Meanwhile, Android tablets – that’s the entire field of Android tablets – will account for about 116 million sales in 2015, Gartner predicts. Tablets powered by QNX, from BlackBerry maker Research In Motion, and Windows operating systems will account for 26 million and 34 million sales, respectively.

In the nearer term, Gartner doesn’t expect anybody in the market to bring anything out that will hold its own against the iPad. In 2012, Gartner predicts 66 million iPad sales and only about 26 million Android tablet sales. The firm also expects Apple to run away with sales this holiday season, which is a pretty fair bet as well.

But while Apple’s iPad is a juggernaut in the tablet sphere, Gartner’s prediction seems to discount another four years of Android device makers working hard to innovate and find a way to topple the iPad. Sure, the iPad had an 18-month lead on its competition and those competitors are still working on a way to really stand against the iPad – but we’re talking about four years of development time here. And we’re also talking about one device versus an entire range of models from several manufacturers.

As GigaOM points out, Gartner also puts a lot of growth behind QNX, expecting 3 million sales of the tablet this year. But in the most recent quarter, RIM shipped only 200,000 of its PlayBook tablet, and only 700,000 or so in the quarter before that. And that’s shipments, not sales, which will likely be even lower.

Gartner’s predictions aren’t exactly crazy, but already Android tablets are getting better and better at mimicking the experience that Apple provides. The Samsung Galaxy Tab 10.1 is adding more and more features and apps all the time (like Netflix), and when Android as a platform catches up a little more with Apple’s iOS, it’ll likely be a very valid competitor.

Given another four years of work, it seems hard to imagine that another company or companies won’t find their own magic tablet bullet, just as Apple has.