LA PAZ, Bolivia (AP) _ The 77-year-old elder stateman who assumed Bolivia's presidency for the fourth time says he will devalue the currency, renegotiate the foreign debt and seek foreign investment to revive South America's poorest economy.

The inauguration of Victor Paz Estenssoro on Tuesday marked the first time in Bolivia's turbulent history that a civilian government turned over power to another civilian government of a different party.

The ceremony took place before a joint session of Congress on the 160th anniversary of Bolivia's independence from Spain.

''Democracy is neither a complete ideal nor a panacea for our faults,'' Victor Paz Estenssoro declared in his inaugural address Tuesday. ''If we do not immediately apply pragmatic, realistic and concrete measures to end the crisis, we run the risk of making Bolivia bankrupt.''

Paz Estenssoro donned the red, green and yellow presidential sash for a four-year term one day after Congress chose him to succeed Hernan Siles Zuazo, a leftist forced by political opposition and the world's highest inflation rate to step down a year early.

The nature of his victory gave Paz Estenssoro, a moderate populist, no better odds of surviving. He received just 26.4 percent of the popular vote, but ruled out any coalitions and filled his 18-man Cabinet with members of his Revolutionary Nationalist Movement.

At the end of his 25-minute televised speech, he told his leftist and rightist foes: ''It's time to join forces. The road is long and full of difficulties. The sacrifice of all is essential.''

Outside the three-story legislative palace, which was ringed by national guardsmen and police, several thousand people packed the Plaza Murillo to hear their new leader's voice on loudspeakers.

They heard the architect of Bolivia's 1952 revolution offer traditional remedies for today's 8,200-percent inflation and a recession that has shrunk the monthly minimum wage from the equivalent of $59 to $10 in four years.

Paz Estenssoro vowed to stimulate exports of gas, tin and other minerals by devaluing the peso, now fixed at 75,000 to the dollar in official trading, to its black market value of around 1 million to the dollar.

Bolivia will renegotiate its $4.8 billion foreign debt to make new loans available, and will seek foreign investment to explore for petroleum before the country runs out of reserves, he said.

Siles Zuazo canceled several oil exploration contracts with foreign companies.

Roberto Gisbert, the new finance minister, said the devaluation will be proposed to Congress within days. He said it will permit Bolivia to negotiate credit agreements with the IMF and private banks holding a third of the foreign debt. Bolivia suspended payments to those creditors in March 1984.

Bolivia's powerful labor movement is expected to resist the devaluation, as are the leftist parties that helped Paz Estenssoro win in Congress over former military ruler Hugo Banzer Suarez.

The Reagan administation sent its ''warmest congratulations'' to the new president but offered little hope for new aid commitments.

Elliot Abrams, assistant secretary of state for inter-American affairs, represented the United States at the inauguration. He said Bolivia could collect $100 million in U.S. aid by moving against illegal cocaine production.

Paz Estenssoro called for a national agreement to reduce the harvest of coca leaves that produce cocaine.

Paz Estenssoro was president for full terms starting in 1952 and 1960. The military deposed him in 1964, just after he was elected for his third term.

Bolivia is one of six South American countries to turn to democracy since 1979, when most of the continent was under military rule.

In a show of support, elected presidents Raul Alfonsin of Argentina, Belisario Betancur of Colombia and Julio Maria Sanguinetti of Uruguay attended the inauguration.