instruction: calculate the number of patients who have received a non-invasive mech vent two or more times until 4 years ago.
response: SELECT COUNT(DISTINCT t1.subject_id) FROM (SELECT admissions.subject_id, COUNT(*) AS c1 FROM procedures_icd JOIN admissions ON procedures_icd.hadm_id = admissions.hadm_id WHERE procedures_icd.icd9_code = (SELECT d_icd_procedures.icd9_code FROM d_icd_procedures WHERE d_icd_procedures.short_title = 'non-invasive mech vent') AND DATETIME(procedures_icd.charttime) <= DATETIME(CURRENT_TIME(), '-4 year') GROUP BY admissions.subject_id) AS t1 WHERE t1.c1 >= 2