It will take 1.5 days x 2 = <<1.5*2=3>>3 days to explore 2 islands.
So, the pirates will walk 20 miles x 3 days = <<20*3=60>>60 miles to explore two islands.
The other 2 islands will be explored by walking 25 miles x 3 days = <<25*3=75>>75 miles.
Thus, the pirates will need to walk 60 + 75 = <<60+75=135>>135 miles to explore the 4 islands.
#### 135