When do they start being explicitly discussed? And by which kind of authors? I know of Buddhists like Dharmottara (and Yāmari, thanks to Eli Franco) and Vedānta ones.

Within Mīmāṃsā, Kumārila at the beginning of the Ślokavārttika, pratijñā section, speaks of content (viṣaya), purpose (prayojana) and connection (sambandha). The absence of the ideal reader is no suprise, since before the end of the first millennium this is often the case.

Within Viśiṣṭādvaita Vedānta, Veṅkaṭanātha at the beginning of his Seśvaramīmāṃsā speaks of content, purpose, ideal reader and seemingly not of the connection, although he might be referring to it by speaking of a pravṛttiprakāra. Hence, the group of four was possibly not yet crystallised?