Am I considered sinful for not paying a specific zakat on 4 trucks that are being paid for in installments and operated by 5 workers, knowing that their income covers salaries, expenses, personal expenditures, and the expenses of relatives and guests, with nothing left for savings?

There is no zakat (definition) due on trucks used for work or rent. However, zakat is due on the remaining rental income or earnings if it reaches the nisab (definition), either by itself or combined with other wealth, and a full year has passed over it. The zakat amount is 2.5%. The recipients of zakat are specified by Islamic law as the poor, the needy, those employed to collect it, those whose hearts are to be reconciled, for freeing captives, for those in debt, for the cause of Allah, and for the wayfarer, as stated in verse 60 of Surat At-Tawbah (definition).