You will have to read Yangong yourself. He is an extremely important Drugpa Kabgyu master who exclaimed "This begger's realization came from Dzogchen" and write one of the defining treatises on Mahamudra and the four yogas of Mahamudra.
I have the Padma Karpo material about four yogas. Gampopa is much clearer.
Namdrol wrote:He considers the four yogas of Mahamudra to be Gampopa's unique system developed out of Gampopa's own experience on the basis of sahaja mahamudra.
That's what Jigten Sumgon says too re Gampopa's own experience. However, that doesn't hold up re Four Yogas because, while Gampopa stated the four yogas in a variety of ways, Mila had a song of four yogas and Saraha has a four yoga-like presentation too. Clearly to me four yogas comes from Saraha and is based on the Hevajra Tantra four moments, four blisses, and all those fours. Basically, based on Gampopa's own experience, he did what Mila told him not to do and that was teach sahaja mahamudra independent of tummo.