By Megan Davies

NEW YORK, March 3 (Reuters) - Private equity firm Carlyle Group [CYL.UL] wrote down the value of the investments in its buyout fund Carlyle Partners IV by 13.8 percent during the fourth quarter, a source who has viewed the figures said on Tuesday.

Carlyle Partners IV is a $7.9 billion fund raised in 2005 for buyout investments in the United States, according to a press release Carlyle issued at the time. Carlyle recently raised $13.7 billion for its buyout fund Carlyle Partners V.

The 13.8 percent figure refers to unrealized -- i.e. not yet sold -- privately held investments. When including publicly traded investments, the writedown was 14.8 percent, the source said.

When including gains from realized investments in companies such as car rental firm Hertz Corp and machinery parts firm AxleTech, the writedown was 8.8 percent, the source said.

Carlyle declined comment on the figures.

Private equity firms are obliged for the first time this year to value their companies as if they were to sell them today, rather than years in the future when they may be sold.

Those that have publicly traded funds, or are themselves listed, report figures to the market. Other private equity firms report numbers direct to their investors.

However, it is hard to do an apples-to-apples comparison of different firms' figures as the methods used to calculate the asset values may not be exactly the same.