What are the four dharma seals?

According to Buddhism: One Teacher, Many Traditions by His Holiness the Dalai Lama:

Cultivating the four establishments of mindfulness and reflecting on their specific and common characteristics enables us to understand the four seals that determine a teaching to be the Buddha's doctrine. These are mentioned in the Samadhiraja Sutra in the Sanskrit tradition: (1) all conditioned phenomena are impermanent, (2) all polluted phenomena are unsatisfactory, (3) all phenomena are empty and selfless, and (4) nirvana is true peace.

Here is the wikipedia article on the mentioned Sutra.

Apparently, this Sutra has been fully translated on http://read.84000.co/, but I haven't found it yet.

Hope this helps!

In Tibetan schools the four seals are usually presented like this:

1. Everything that is conditioned (or "composite") is impermanent.
2. Everything that is stained (or "tainted") brings suffering.
3. All phenomena are empty and devoid of a self (or "of independent entity").
4. Nirvana or ("The Realization") is peace.

The first three look like paraphrases of the original Three Marks of Existence:

1. anicca vata sankhara (all assemblies are transitory)
2. sabbe sankhara dukkha (all assemblies are faulty / entail pain)
3. sabbe dhamma anatta (all phenomena are no-self or "coreless")

And here is a commentary by H.H. Dalai Lama from "The Gelug/Kagyü Tradition of Mahamudra."

[For non-Mahayana Buddhists]

1. never separating the actions of their body, speech and mind from the behavior shaped by the vinaya rules of ethical discipline is karmamudra, the seal of their behavior.

2. The realization of the lack of any true, unimputed identity of a person, specifically the total absence or voidness of a person existing as a substantial entity able to stand on its own – the coarse identitylessness of a person according to the prasangika-madhyamaka presentation – is dharmamudra, the seal of their preventive measures.

3. The state of separation from disturbing emotions and attitudes, on the basis of the abiding nature of purity, attained through meditation on the identitylessness of a person is samayamudra, the seal of their close bond.

4. The nirvana or total release that they attain without any remainder of aggregates is mahamudra, their great seal.

[For Mahayana Buddhists having taken the vow of Bodhisattva]

1. never separating the actions of their body, speech and mind from behavior based on practice of the six far-reaching attitudes is the seal of their behavior.

2. Based on their practice of the six far-reaching attitudes in general, and particularly of the joint path of shamatha yoked with vipashyana, their meditation on voidness, free from all mental fabrication and fabricating, seeing that all phenomena appear but without true, inherent existence, like illusion, is the seal of their preventive measures.

3. Through the ethical self-discipline of a bodhisattva, restraining themselves from all selfish thought or action so that they are never tainted by any stain of working for their own purposes is the seal of their close bond.

4. Having followed a path that has had as its essence the combination of compassion and the understanding of voidness, on which they have practiced method and discriminating awareness without one having ever been missing, but rather with method always apprehended within the context of wisdom and wisdom within the context of method, their single taste of compassion and voidness is their great seal. At that point, their ultimate combined vision of both the full extent of what exists and how it all exists, being of a single taste of compassion and wisdom, totally pacifies the extreme of remaining passively in a nirvanic state of total release from all suffering.

I do not know the original/earliest source of these.