Six and not five men may have been responsible for the throat slashings and robbery of four persons in a Fairfax County apartment last month, police said yesterday.

Capt. Jack Zelaska said new evidence showed that four suspects entered the apartment of the victims at 9136 Barrick St. and two others waited outside in a car as the victims were robbed and their throats slashed Aug. 28. At a hastily called news conference yesterday, Zelaska said the motive was simply robbery, not drugs, as had earlier been suspected by police and reported by a county police spokeswoman.

Two suspects have been arrested and are being held without bond in the Fairfax County jail. Andrew E. Porter, 19, of 2632 Columbia Pl. in Landover was extradited yesterday to Fairfax County from Prince George's where he was arrested Saturday.

Another suspect, Phillip A. Boyd of Vienna, was arrested Friday at 9925 Corsica St., Vienna, at the home of a relative. Boyd carried identification stating that he was 18 years old, police said, but they now suspect he is younger. Papers filed in the county General District Court, not juvenile court, as would normally be the case with a youth under 18, listed Boyd's birthdate as Dec. 15. 1958. Boyd's relatives also said he was 18 although Zelaska said yesterday Boyd is 17 and that he lied to police about his age. A police official said later they are trying to obtain Boyd's birth certificate to determine his age.

Both Boyd and Porter have been charged with attempted murder, attempted abduction, armed robbery and use of a firearm during the commission of a felony.

According to police, four men entered the Barrick Street apartment about 2 p.m. Aug. 28, robbed the victims of cash, watches and rings, then forced the four victims into four separate areas of the apartment. The assailants bound the victims' hands, wrapped towels around their heads and slashed their throats police said.

The victims are Stephen Dowell, 20, of the Barrick street address: Laura Bombere, 18, of 3139 Highland La., Fairfax: Debra Bombere, 18, of 3139 Leeds Rd., Oakton, and David C. Perash, 20, of Hanover, Pa.

Zelaska said yesterday that Boyd's mother lived across the hall from where the slashings and robbery took place and that Boyd had been in the apartment several times before the incident.