The Portland Symphony Orchestra and the union representing its musicians have reached a four-year contract that includes a pay increase of 8 percent over the contract’s first two years, the orchestra’s executive director announced in a press release Monday. The contract was accepted unanimously.

The contract includes a 4 percent increase in pay, effective immediately, with the orchestra committing to another 4 percent in the contract’s second year. The contract will be revisited in each of the final two years. The increase in pay covers all rehearsals and performances. Musicians who appear on behalf of the PSO in community performances, lectures and other community roles also will receive pay increases.

Such accord is unusual among American orchestras. At least four U.S. orchestras – in Pittsburgh, Philadelphia, Fort Worth, Texas, and the Pacific Symphony in Orange County, California – are either on strike or working without a contract.

In Portland, most musicians work part time for the PSO, juggling several jobs. Many live in Massachusetts and perform in Boston. “As they must sometimes choose between playing with one ensemble or another due to schedule conflicts, it is important that the PSO’s wages are as competitive as possible,” PSO executive director Carolyn Nishon said in the press release. “We are fortunate that all parties involved in the process aspire to see the PSO become a world-class orchestra and understand the need to support our musicians in accomplishing that.”

Robert Couture, the union representative and vice president of the Boston Musicians’ Association, which is part of the American Federation of Musicians, praised the partnership among musicians and the orchestra’s management and board. “We look forward to a bright future,” he said in the press release.

Russell Wilson, a viola player and chairman of the musician’s committee, praised the orchestra for its “continued commitment to supporting its musicians. … With this contract, the Portland Symphony Orchestra is reaffirming its commitment to provide high-quality performance and music education to the Portland community.”

The PSO has an annual budget of about $2 million.