Four Japanese subcompacts were among the five 1978-model cars with the worst records for injury-causing-accidents, according to a study released yesterday by the insurance Institute for Highway Safety.

The fifth car was the American-made Ford Mustang II.

In contrast, the five cars in which the driver is least likely to be injured were all comparatively large General Motors four-door models.

Topping the cars with the worst injury records, based on the number of insurance injury claims filed through December 1978, were, the Honda Civic, Toyota Carolla, Datsun B210, Ford Mustang II and Toyota Celica.

The cars involved in the fewest accidents resulting in injury claims were the Oldsmobile Delta 88, Chevrolet Caprice, Buick LeSabre Chevrolet Impala, and Chevrolet Nova.

"These results, added to the large body of prior evidence, show that there abviously are huge differences in degree of crash protection manufacturers built into different automobiles," said IIHS president William Haddon in releasing the results of the study here.

The study also indicated that as the size of the car decreases, injury claim frequencies "increase dramatically, especially with young drivers.

Clarence Ditlow, executive director of the Center for Auto Safety, said yesterday that despite the study's findings, "small cars are not necessarily unsafe.

"A manufacturer can choose to design safety into a small car," Ditlow added. "What the study shows is that the majority of manufacturers have chosen not to do so." He recommended that consumers, if they want to purchase a small car, "choose small cars that are shown to be safer, like the Chevrolet Nova."

Younger drivers, no matter what size the car they were driving, were found to have higher injury claim incidences than older drivers.

The study also found that two-door cars "have consistently shown higher injury claim frequencies than corresponding four-door models."

And sports and specialty models in the subcompact, compact and intermediate size groups consistently had the highest injury claim records, the IIHS reported.

The study evaluated results from 41 vehicle series from 13 automobile makes, those with more than 5,000 insured vehicle years of exposure. Other cars did not have enough available data for adequate sampling.