The first major postal strike in U. S. history entered its fifth day yesterday as President Nixon ordered 2500 Federal troops into New York City to help process strike-bound mail.

Declaring a national emergency, Nixon said yesterday in a televised statement. "What is at issue here is the survival of a government based on law," and urged the estimated 167,000 postal workers still on strike "to return to [their] jobs so that negotiations can begin."

The central issue in the strike is the National Letter Carriers Union demand for a pay scale that will increase from $8500 to $11.700 in five years. At present the scale starts carriers at $6176 a year and raises them to $8442 after 21 years.

Boston's OK

Irving Shear. spokesman for the Boston Postal District, said yesterday that only four of the area's 90 stations did not report to work. Members of the Boston branch of the Massachusetts Letter Carriers Association voted at a tumultuous meeting Sunday night to return to work after a brief stoppage Saturday.

Shear said that-except for the four striking stations in Belmont, Newtonville, Newton Center, and Auburnville-the only effect the strike will have is on mail destined for metropolitan New York. This mail, he said, will be held until the strike is over.

SDS held a rally in support of the striking postal workers at noon yesterday,