U.S. airlines operating planes with more than 30 seats posted their lowest accident rate ever in 1984 while the number of fatalities increased sharply for commuter airlines operating planes with 30 or fewer seats, the National Transportation Safety Board said yesterday.

The board, in releasing its annual aviation statistical summary, said there were four fatalities in the big-plane category in 1984, which ties it with 1981 as the second-safest year. In 1980 there were no big-plane fatalities on U.S. airlines.

The record-low accident rate last year for big planes was .164 for every 100,000 flight hours. The previously best year was 1980, when there were .221 accidents for every 100,000 flight hours.

There were 12 accidents on big-plane U.S. airlines last year; the only fatal accident occurred on a Zantop International Airlines cargo flight that crashed May 30 in Pennsylvania and killed all three crewmen and a passenger.

Small-plane or commuter airlines showed a minimal increase in their accident rate over 1983, when there were 11 fatalities in 17 accidents and a rate of 1.13 accidents for every 100,000 flight hours.

Last year, scheduled commuter airlines had 41 fatalities in 20 accidents. The rate was 1.14 accidents for every 100,000 flight hours. The commuter accident rate continues generally downward: It was 2.08 in 1982 and 5.13 in 1975.

Nonscheduled commuter carriers, known as on-demand air taxis, had 141 accidents in 1984 with 55 fatalities. Both the accident rate and number of fatalities were improvements over 1983.

General aviation -- which includes everything from the smallest propeller plane to a complex business jet -- had aviation's highest fatality count, as it always does, although for the first time fatalities were below 1,000 at 998. The rate was 9.82 accidents for every 100,000 flight hours.