With 4.1 million Macs sold in Q4 2010, Apple controlled 4.42 percent of the worldwide market for PCs in the fourth quarter.
With 14.4 million Macs sold in CY 2010, Apple had worldwide marketshare of 4.13 percent for all of 2010.
It's worth noting, however, that Apple sold a phenomenal 7.33 million iPads in Q4 2010 as well, easily beating the most optimistic predictions. And while the iPad isn't a PC, it essentially will be soon. If you compare the iPad to the wider PC market (adding iPad sales to PC sales), with just this device, Apple controlled 7.7 percent of the market for PCs in Q4 2010 (worldwide).
Not interesting enough? How about this one: If you aggregate Mac and Apple sales (12.12 million units), then Apple controlled 11.77 percent of the worldwide market .. for PCs. Note, however, that his doesn't take into account sales of similar devices like the Samsung Galaxy tab. But there is little doubt that Mac + iPad unit sales in Q4 2010 represented roughly 10+ percent of the sales for all PCs and similar computing devices. That is simply amazing and should give pause to PC makers everywhere.
Related side-note: So Apple sold 12.12 million PC- and PC-like devices in Q4 2010. How does this compare to the biggest PC makers in the world? Quite favorably. HP is still number one by a wide margin, with 17.77 million PCs sold worldwide. But Apple is neck-and-neck, or even above, PC makers like Dell (10.9 million units) and Acer (10.8 million units). Again, this is a bit skewed because it doesn't count other tablet device sales, but whatever. Depending on how you look at it, Apple is the number two PC maker in the world, arguably, and certainly in the top five. This is the first time such a thing has happened, and it's only because of the iPad; previously, Apple didn't even register in the top five worldwide, ever.
Honestly, the big news here is the surge in iPad sales. Few in the Apple-friendly press seem to remember this, but iPad sales actually disappointed in Q3 2010, when actual unit sales were well under all estimates. Sales rebounded--and then some--in Q4, raising the yearly total dramatically and raising estimates for the future as well.
Impressive stuff no matter how you slice it.
(Any math errors are mine, and inadvertent. If you see something fishy, let me know. My calculator skills are lacking, but this is pretty simple math. Remember that I always use an average of figures from IDC and Gartner for all calculations. Remember too that Apple does not release US-specific sales figures, making US market share calculation more black art than mathematical.)