Prime Computer reported a 50% increase in income on a 16% rise in revenue for its fourth quarter ended Dec. 31. The Natick, Mass.-based computer manufacturer had net income of $21.3 million, compared to $14.2 million a year before. Revenue rose to $267 million from $230.9 million. For the full year, Prime's net income totaled $64.8 million, up 38%. Revenue rose 12% to $960.9 million. The firm said its results were buoyed by strong sales of its 50 Series computers and increased demand for its products in international markets.