Yes, at that point you would be loading your data against UD2 members. Your UD4 member would then automatically derive its data, when you look at it in a Cube View or report, by aggregating all the records of the UD2 members that have "ABC" in their Text1 property (where members of the other dimensions are the same, i.e. the same combination of Scenario/Entity/Time/etc etc).
Note that those UD4 values would not be saved anywhere, they are produced dynamically at display time.
The "as Attribute" feature is effectively a way to analyse data by certain elements, without actually storing any data that specifically points at such elements - because you know there is a direct relationship between the elements and metadata members in existing dimensions (they are an attribute of other members), so the system can calculate the relevant sums for you.
Note that this feature should not be confused with Attribute Dimensions as specified in the Integration tab of Cube Properties. That's a different thing.