The concept that we are discussing here is that "which" modifier in this sentence can either refer to x or y in the construction x of y. In the original sentence (in the question), the which modifier refers to y - union since the context requires that.
Then we discussed the reworded sentence - which consist of 4 people from each city. The poster wanted to find out if this modifier can modify "representatives". For this we discussed the standalone sentences and determined that this modifier cannot modify "representatives' since the context does not permit that - representatives do not consist of 4 people. Thus the reworded sentence is incorrect. (note that we cannot even use "which" to refer to "representatives").
So that's it about the discussion here.
Now with all this discussion about what the modifier can modify, lets consider one more example in which the context requires the modify to modify x = representatives.
The newly elected baseball commissioner has asked that a federal arbitrator would mediate negotiations between representatives of the umpire's union, who flew from 4 metropolitan cities for a limited time only, and the lawyers representing major league franchise owners.
Here the modifier modifies "representatives" and not just the "union".
Take-Away - Understand the context of the sentence to see the intended reference of the modifier.
Hope this helps. Let me know if you have any questions.
Payal
