Thanks from me too! Would You happen to know if there are whole sanskrit sadhanas existent, and if they contain the four limitless minds, like most of the later sadhanas do? I have seen only partial sanskrit sadhanas, that are mainly from Sadhana Mala, in Bhattacharya's Indian Buddhist Iconography.
Sure -- for example, there is a complete sadhana in Sanskrit and translation in the Vajrayogini book by Elizabeth English, has the brahmaviharas too.
But if you look at it, you will notice a slight difference between the typical Sanskrit and the typical Tibetan sadhana -- the Tibetan ones are complete liturgies to be recited from beginning to end ("I arise from the syllable x as the deity y"), the Sanskrit ones tend to be worded more like a manual ("now the yogin should arise from the syllable x as the deity y"). So you may have to look around a bit if you want to find a nice Sanskrit verse for the four immeasurables to be recited as part of a liturgy.