Jodi starts off walking 1 mile a day for 6 days a week.  On the second week, she walks 2 miles a day, 6 days a week.  On the third week, she walks 3 miles a day, 6 days a week. Finally on the fourth week, she walks 4 miles a day, 6 days a week.  How many miles has she walked in 4 weeks?