G>>from n in service.Fun()
G>>from _2 in Observable.If(n%2 == 0, service.Action2(), Observable.Empty<Unit>())
G>>from _3 in Observable.If(n%3 == 0, service.Action3(), Observable.Empty<Unit>())
G>>from _4 in Observable.If(n%4 == 0, service.Action4(), Observable.Empty<Unit>())
G>>select "Completed";
G>>

Observable.Zip(
    Observable.FromEventPattern<Args>(obj, "EventName"), 
    Observable.Start(() => obj.Method(params),
    (evt, _) => selector(evt))

var funTask = TaskFactory.FromAsync(service.BeginFun, service.EndFun, null);
var action1Task = funTask.ContinueWith(task => task.Result % 2 == 0 ? TaskFactory.FromAsync(service.BeginAction1, service.EndAction1, null) : task).Unwrap();
var action2Task = funTask.ContinueWith(task => task.Result % 4 == 0 ? TaskFactory.FromAsync(service.BeginAction2, service.EndAction2, null) : task).Unwrap();
var action3Task = funTask.ContinueWith(task => task.Result % 8 == 0 ? TaskFactory.FromAsync(service.BeginAction3, service.EndAction3, null) : task).Unwrap();
Task.WaitAll(action1Task, action2Task, action3Task);

при использовании Windows Phone

static MyDelegate MakeAction2(Sevice sevice, int n) {
  return (o, eventArgs1) => {
    if (n % 2 == 0) {
      service.Action3Completed += MakeAction3(service, n);
      service.StartAction2();
    }
////////////// Добавилось ////////////////
    else
       MakeAction3(service, n);
//////////////////////////////
  }
}

service.FunCompleted += (sender, eventArgs) => { 
  int n = eventArgs.Argument;

  if (0 == n%2)
    MakeAction2(service, n)(sender, eventArgs); 
  else if (0 == n%3)
    MakeAction3(service, n)(sender, eventArgs); 
  else if (0 == n%4)
    MakeAction4(service, n)(sender, eventArgs);
}

static MyDelegate MakeAction2(Sevice sevice, int n) {
  if (n % 2 == 0)
    return (o, eventArgs1) => {
      service.Action2Completed += MakeAction3(service, n);
      service.StartAction2();
    }
  else
    return MakeAction3(service, n);
}

// и на верхнем уровне совсем просто:
var service = new Service();
service.FunCompleted += (sender, eventArgs) => { MakeAction2(service, eventArgs.Argument)(sender, eventArgs); };
service.StartFun();