The first two tanks need 8 x 2 = <<8*2=16>>16 gallons of water.
The other two tanks need 8 - 2 = <<8-2=6>>6 gallons of water each.
So, the two other tanks need 6 x 2 = <<6*2=12>>12 gallons of water.
Nicole needs 16 + 12 = <<16+12=28>>28 gallons of water every week for her four tanks.
Therefore, she needs a total of 28 x 4 = <<28*4=112>>112 gallons of water in four weeks.
#### 112