In instruction I2 we are writing the computed sum value to the register $2 → this counts as Write operation.
In instruction I4 we’re writing/storing the value present in register $2 to the address value wherein Address = $4 + 20. To do this we will have to read the contents present in the register $2 which counts as a Read operation on register $2.
Please also note that the value in $2 is not overwritten between instructions I2 and I4 hence I2-I4 is a valid RAW (Read after Write) dependency.
