Coal miner Alpha Natural Resources Inc posted a fourth-quarter loss, partly because some customers switched to cheaper natural gas for power generation.

Alpha's stock fell 4 percent to $19.00 in premarket trading.

The net loss was $733.3 million, or $3.34 per share, compared with net income of $10.8 million, or 9 cents per share in the same quarter of 2011, the Abingdon, Virginia-based company said on Friday.

Excluding a goodwill impairment charge of $745 million and other items, the adjusted loss from continuing operations was 7 cents per share.

(Reporting by Steve James in New York)