DJCC
737-900ERs have floor level emergency exits aft of the wing in addition to the 4 overwing hatches and 4 doors of the -800. These doors and the redesigned concave aft pressure bulkhead allow the 900ER to carry a greater load than the original 900 was certified (max 189) and thus attractive to LCCs.
In A
VB configuration they are likely to carry over 200 pax so a 5th crew member would be necessary.