Let $y_i$ be the contribution of agent i towards the production of $y$
So when $y=4$
$\Rightarrow \sum y_i=16 \Rightarrow \sum x_i = 14$
The set of pareto efficient allocations is $...
Most of the discussion on Pareto optimum misses the very basis of the model. It is simply a model describing how a set of producers and consumers moves to an equilibrium in a given market situation.