The owner of a temporary Massachusetts employment agency pleaded guilty Monday in federal district court to hiding $10 million in company assets and filing false income tax returns over a four-year period by using a Worcester-based check cashing service.Su Nguyen, 60, of Quincy, the owner and operator of General Employment Services, pleaded guilty to aiding and assisting the filing of false tax returns, according to the office of Acting U.S. Attorney Joshua S. Levy. Scheduling is sentenced for Sept. 5.According to court records, Nguyen deposited some of the checks his company received as payment for services into a corporate bank account. That was the income he reported to the Internal Revenue Service. Nguyen cashed the majority of checks made out to his company as payment for services at a check-cashing service in Worcester.Prosectors say Nguyen used the funds on himself and to pay some of his employees’ wages. The business owner cashed checks worth more than $10 million over four years between 2016 and 2020, Levy’s office said.The IRS calculates he failed to pay over $2 million in personal income and corporate taxes.