Approach Controller experienced a loss of separation event when allowing traffic to enter a MVA area below authorized altitude; the reporter noting his existing mindset as flawed.

Narrative

Aircraft X; an IFR [flight] inbound from over ZZZ VOR at 7;000 FT checked on my frequency. I instructed aircraft X to descend and maintain 5;000 FT and to proceed direct ABCDE intersection for [the] RNAV approach into ZZZ1. Approximately 10 NM north of ABCDE; I instructed aircraft X to cross ABCDE at or above 4;000 FT; [and] cleared [for the] RNAV approach to ZZZ1. Aircraft X was in a 4;700 FT MVA. The OSIC was observing from the adjacent RADAR position; [but] then left the room. Aircraft X penetrated the 4;700 FT MVA in his descent to 4;000 FT. We had been working a series of days with good VFR weather and my mindset was in the area of clearing for visual approaches regardless of altitude. Got my thinking backwards and crossed up; even when I took a second to rethink the approach clearance; thinking; 'oh yeah at or above 4;000 FT will be just fine.' My endeavor to hurry and issue clearance without taking another second to rethink the clearance resulted in the error. There was no reason for me to hurry as traffic was very light and not complex. Just goes to show how we can get comfortable with a set of rules/procedures; several days of VFR weather; then a day of VFR but low hanging clouds; and make knee jerk reactions.