Ceradyne Inc., Costa Mesa, reported a loss for 1988 of $6 million, compared to a loss of $5.5 million a year earlier. Sales were virtually unchanged at $25.6 million. For the fourth quarter, the company reported a net loss of $3.4 million, compared to a net loss of $2.9 million for 1987. Sales rose 8% to $6.4 million from $6 million. The company said the fourth-quarter loss included asset write-downs related to the reduced level of defense-related business and the establishment of reserves for product and facility consolidations. Ceradyne manufactures ceramic products and components for defense, aerospace and industrial applications.