A report released by Gartner places Apple in fourth place for U.S. and worldwide PC shipments during the fourth quarter of 2008. Apple’s market share was at 8.0%, up from 6.7% the previous year, but down from 9.5% in the third quarter of 2008.

The decline in Apple’s market share is not totally unexpected, considering the state of the economy and taking into account that the third quarter is the “back to school” quarter, where Apple tends to come out on top.

Apple’s computer shipments have also only grown 8.3% since the fourth quarter of 2007, compared to the usual 30-40% year-over-year. Fortunately, Apple continued to outperform the industry as a whole. However, the company’s growth was overshadowed by PC manufacturers Acer, which saw a year-over-year increase of 55.4%, enough to steal Apple’s third place position in the industry. Gartner attributes Acer’s success to their netbook lineup.

IDC also released a similar report with parallel results. Apple’s fourth quarter market share ranked fourth at 7.2%, while computer shipments grew 7.5% over 4Q 2007. IDC’s US market share data for 2008 also revealed that Apple placed fourth with a 7.7% market share, up from 6.2% from 2007.

Apple was unable to break into the top give vendors for worldwide market share in 2008.