To arrange these numbers in order from least to greatest, it's important to understand each number and where it falls on the numerical scale.
1. √4 is the square root of 4. The square root of four is 2.
2. -1.5 is a negative number.
3. -8/5 which equals to -1.6, which is another negative number on the scale.
4. 0.8 is a positive fractional number.
5. √2 is the square root of 2, which is approximately 1.41.
Arranging these numbers from least to greatest gives us the sequence: -1.6, -1.5, 0.8, √2 (or 1.41), and √4 (or 2).