Here are the four (Unbinding) Nibbana Suttas:
All four suttas begin thusly:
I have heard that on one occasion the Blessed One was staying near Sāvatthī at Jeta's Grove, Anāthapiṇḍika's monastery. And on that occasion the Blessed One was instructing, urging, rousing, & encouraging the monks with Dhamma-talk concerned with unbinding. The monks — receptive, attentive, focusing their entire awareness, lending ear — listened to the Dhamma.
I'll post <...> in the following suttas for the above part.
Nibbāna Sutta: Unbinding (1)
<...>
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
There is that dimension, monks, where there is neither earth, nor water, nor fire, nor wind; neither dimension of the infinitude of space, nor dimension of the infinitude of consciousness, nor dimension of nothingness, nor dimension of neither perception nor non-perception; neither this world, nor the next world, nor sun, nor moon. And there, I say, there is neither coming, nor going, nor staying; neither passing away nor arising: unestablished,[1] unevolving, without support [mental object].[2] This, just this, is the end of stress.
Nibbāna Sutta: Unbinding (2)
<...>
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
It's hard to see the unaffected, for the truth is not easily seen. Craving is pierced in one who knows; For one who sees, there is nothing.
Nibbāna Sutta: Unbinding (3)
<...>
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
There is, monks, an unborn[1] — unbecome — unmade — unfabricated. If there were not that unborn — unbecome — unmade — unfabricated, there would not be the case that escape from the born — become — made — fabricated would be discerned. But precisely because there is an unborn — unbecome — unmade — unfabricated, escape from the born — become — made — fabricated is discerned.[2]
Nibbāna Sutta: Unbinding (4)
<...>
Then, on realizing the significance of that, the Blessed One on that occasion exclaimed:
One who is dependent has wavering. One who is independent has no wavering. There being no wavering, there is calm. There being calm, there is no yearning. There being no yearning, there is no coming or going. There being no coming or going, there is no passing away or arising. There being no passing away or arising, there is neither a here nor a there nor a between-the-two. This, just this, is the end of stress.[1]
The first sutta seems to be talking about beyond the major jhanas.
The second sutta: Sounds like Tao Te Ching 1, no?
Third sutta: sounds very much like the Zen "pure consciousness," which is spontaneous, no?
Fourth sutta: Finding rest in non-duality?