It depends on your climate, you might do an energy audit or just switch to electric, it doesn't seem too out of line for four units though $35 ish dollars per month depending on the size of the unit and how much they use the climate control.
My single family rental in Cleveland is probably 100 bucks a month in gas during the winter
Is the gas usage reasonable? Could there be a leak or something wrong with the way the meter was read? Maybe estimated instead of read?