Andrew's family consists of 1 + 2 + 2 = <<1+2+2=5>>5 people.
Each of Andrew's family members uses 100 masks / 5 = <<100/5=20>>20 masks.
With everyone wearing a mask every 4 days, the masks will last 20 masks * 4 days/mask = <<20*4=80>>80 days.
#### 80