Sec. 4. RETIREMENT SYSTEM AND TRUST FUND. A fire fighters' retirement system and trust fund are established in each municipality or other political subdivision to which this Act applies. The board of trustees of each retirement system established by this Act shall hold or cause to be held in trust the assets appropriated or dedicated to the system or fund, separate from other money or accounts administered by the board of trustees or the municipality or other political subdivision, for the exclusive benefit of the members and retirees of the system and their beneficiaries.