what is the citation of that piti sukha simile for the 4 jhanas from?
The citation is from the Visuddhimagga, used to explain piti (rapture) is mental formation (sankhara) rather than a feeling (vedana). Here, the Visuddhimagga says piti is something mental rather than originating from the physical; thus appears to contradict the meaning of 'rupa jhana' and contradicting the sutta terms "vedanāsu vedanānupassī" and "cittasaṅkhārapaṭisaṃvedī" (where 'cittasaṅkhāra' means feelings, per MN 44).
might this sutta, MN 40, be what inspired that simile for jhana piti/sukha differentiation? Should the words piti sukha appear in MN 40 lotus pond, but got lost in transmission?
MN 40 is not about jhana. While, similar to the Visuddhimagga, the pāmojjaṃ (joy) & pīti (rapture) in MN 40 appear to be born of mental formations (sankhara), as said, MN 40 is not about jhana where as Visuddhimagga claims to be about jhana. Therefore, there seems to be a contradiction here between MN 40 and Visuddhimagga. However, if the writer of the Visuddhimagga, similar to FrankK, misinterpreted MN 40, then yes, MN 40 might be what inspired that (false) simile for jhana piti/sukha differentiation
What does the MN 40 lotus pond simile mean on its own terms, just for this sutta? What are the 4 directions referring to?
MN 40 does not appear to be about jhana. MN 40 describes the mental joy of knowing or recognising the mind is pure; similar to the worldly idea of "self-esteem". Using AN 11.2 to explain, MN 40 is describing the joy arising from an absence of remorse.
However, the rapture of jhana is unrelated to any mental recognition, judgment, knowing or perception. The rapture of jhana is "physical" in its origins and is born from the stress in the physical body being thoroughly calmed & dissolved (to the degree that the mind can longer feel/discern the physical body, which consumates the 1st jhana, in which there is no knowing of the physical body, as explained by Bhikkhus Buddhadasa, Brahmavamso, Sujato, etc).
I recall I have explained before the meaning or types "rapture" ("piti") are not always the same. There are at least five different types of rapture found in the suttas, namely:
Rapture when hearing about there is a path to an end to suffering (SN 12.23).
Rapture when free from remorse and when feeling morally righteous (AN 11.2; MN 40).
Rapture when first successfully having mindfulness & insight (SN 45.3; rapture factor of enlightenment).
Rapture of neighbourhood concentration (MN 118)
Rapture of attainment concentration/jhana (MN 4; MN 19; MN 111).