Soviet leader Konstantin Chernenko, whose illness has kept him out of public view for the past four weeks, renewed his call for a halt in the arms race today and said only "concrete steps" in that direction will make it possible to "do away with the fear of the future."

The 73-year-old president made the remarks in a letter to an 18-year-old Canadian high school student, Laurie Piraux of Calgary, Alberta.

The text of Chernenko's letter was distributed by the official news agency Tass along with the text of Piraux's letter to Chernenko, in which she stated that a study project about the prospect of a nuclear war had made her "feel insecure."

Today's reply follows a series of messages from Chernenko in recent days, apparently designed to provide public reassurances on the condition of his health.

There have been reports that Chernenko was hospitalized with a respiratory ailment in early January. He was last seen in public Dec. 27, when he awarded medals to several prominent literary figures. The next day it was announced that he would attend a Warsaw Pact summit in mid-January in Bulgaria. The summit was postponed indefinitely Jan. 15.

Soviet authorities also have postponed next month's scheduled visits to Moscow of former West German chancellor Willy Brandt and the new French foreign minister, Roland Dumas. The February visit of Greek Prime Minister Andreas Papandreau has not yet been postponed, however.

Tonight, Tass reported that Chernenko spoke to a meeting of the ruling Politburo today.

Chernenko's letter to the Canadian student recalled the use of the same device by his predecessor, Yuri Andropov, who publicized his reply to an 11-year-old schoolgirl from Manchester, Maine, in late April 1983.

Andropov's letter was sent a month after he became seriously ill.

Like Andropov's letter, Chernenko's reply cast Moscow's peace drive in the simplest terms possible to reach the widest audience.

Piraux had solicited Chernenko's view on the subject and wondered whether Soviet teen-agers were also questioning the current course of events.

The Soviet leader said that young generations in this country are "convinced" that the international community was capable of "resisting the war danger." The essence of Piraux's letter, Chernenko said, was the question of what could be done to secure peace.

"The answer is simple," he continued. "It is necessary to observe norms of intercourse between states and peoples, to develop relations between them on the basis of equality and noninterference in internal affairs. It is necessary to renounce forever the use of force or the threat to use it.

"It seems to me that you view the following question in your letter as particularly important: 'Why cannot all people, all races, live in a world of peace? Why is there such a power struggle?' I will give you a definite answer: nobody can achieve power over the entire world but attempts to carry out such a mad plan may cost the whole of mankind very dearly."