Medium Transport; High Wing; 2 Turboprop Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 27.0
Distraction; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
Procedure; Airspace Structure; Chart Or Publication; Human Factors
Aircraft X checked in descending to 7000 feet with restriction to cross arrival fix at 7000 feet. I didn't need the aircraft to cross the fix at 7000 feet so I intend to give the aircraft a descent to 6000 feet without restrictions. The Minimum vectoring Altitude (MVA) is 5700 feet; the next MVA is step down is 4000 feet. I should have cleared the aircraft to descend to 6000 feet. I cleared the aircraft to descend to 4000 feet and the aircraft read back 4000 feet. I did not realize that I said 4000 feet when I meant 6000 feet. I notice aircraft is descending below 6000 feet. The aircraft was at 5600 feet and almost leaving the 5700 foot MVA area. I stopped  aircraft at 5000 feet and  the pilot complied. I did not climb the aircraft as the aircraft was leaving a MVA area and higher terrain was behind aircraft.I'm not sure how to prevent this. I have descended aircraft in this area to the correct altitude hundreds of time. I was not busy or distracted. The aircraft was the only one on frequency and had my attention. The wrong words came out and I did not catch it.
A Tracon Controller reported inadvertently descending an aircraft below the Minimum Vectoring Altitude.