Pareto optimality when the allocations are restricted
Let me elaborate on the comments quickly - maybe only for the sake of this question not being "unanswered".
Pareto optimality only makes sense when defined with respect to feasible allocations. That is, your basic definition above lacks an important word:


  an allocation is PO if there does not exist another feasible
  allocation in which all players are weakly better off and at least one
  player is strictly better off.


Without this word, you can easily find an allocation that dominates "A gets slices 1&amp;3; B gets slices 2&amp;4". For instance, you could give Alice eight additional full cakes of chocolate. However, there are no other cakes and, hence, this allocation is not feasible and therefore does not Pareto dominate what you suggest.

As a result of your feasibility restriction, any allocation in which one agent gets one slice of the preferred kind and the other one gets the other three is PO.
If A gets the entire cake, she can make B better off by giving him a slice of V which she does not care about; analogously if B has the whole cake. If both agents get two connected slices, a similar improvement exists.