You can also do this with the Calculator
step. The advantages are that it doesn't require dropping into code, thus maintaining the visual metaphore, and it's faster than JavaScript.
In the calculator step, when you define a new field, that field becomes available to subsequent calculations within the same step. So if I have Field_000 ... Field_003, I can configure the step like this:
The output from the calculator step will have two new fields, 'tmpField1' and 'outField' which has the concatenation of the four fields.
BTW, if by 'populated' you mean the field has a non-null value, you could use the calculator's NVL function to achieve the same thing. That would essentially be a Coalesce
operation.