Splitting Observable into windows - system.reactive

Question

I am looking for Observable.Window like operator with opening window selector
As example we can take a sequence of natural numbers.
I am wondering how to split this sequence into windows so every new window starts if number is greater than 4 or window size has reached 3
Input sequence is IObservable<int>
Output sequence is IObservable<IObservable<int>>
Sequence 1 2 5 3 1 1 2 3 1 5 0
will produce windows 1 2; 5 3 1; 1 2 3; 1; 5 0

Answer 1

Using C# this works:
var observable =
source
.Concat(Observable.Return(-1))
.Publish(sp =>
sp.Zip(sp.Skip(1), (x0, x1) => new { x0, x1 })
.Publish(zsp =>
zsp
.Window(zsp.Where(x => x.x1 >= 4))
.Select(xs => xs.Select(x => x.x0).Window(3))
.Merge()));
I get this result:

Answer 2

As per my comment on the question I am unsure whether the sequence 1, 2, 3, 5 should produce the windows 1 2 3 | 5 or 1 2 3 | | 5 (see the empty window which is what Enigmativity's answer produces). My answer does not produce the empty window:
public static IObservable<IObservable<T>> Window<T>(this IObservable<T> source, Func<T, bool> predicate, int maximumWindowSize)
{
return Observable.Create<IObservable<T>>(obs =>
{
var currentWindow = new Subject<T>();
obs.OnNext(currentWindow);
var count = 0;
return source.Subscribe(x =>
{
if (count == maximumWindowSize || predicate(x))
{
count = 0;
currentWindow.OnCompleted();
currentWindow = new Subject<T>();
obs.OnNext(currentWindow);
}
currentWindow.OnNext(x);
count++;
}, obs.OnError, () =>
{
obs.OnCompleted();
currentWindow.OnCompleted();
});
});
}
This can be used like so:
var windows = source.Window(x => x > 4, 3);