Ayaṃ pana viseso, yasmā sukhavedanā adukkhamasukhāya vedanāya āsevanapaccayena paccayo na hoti, catutthajjhāne ca adukkhamasukhāya vedanāya uppajjitabbaṃ, tasmā tāni upekkhāvedanāsampayuttāni honti.
Ñ(IV,182): But there is this difference: blissful (pleasant) feeling is not a condition, as repetition condition, for neither-painful-nor-pleasant feeling, and [the preliminary work] must be aroused in the case of the fourth jhāna with neither-painful-nor-pleasant feeling, consequently these [consciousnesses of the preliminary work] are associated with neither-painful-nor-pleasant feeling,
Sayādaw U Sīlānanda: [
[the preliminary work] must be aroused in the case of the fourth jhāna with neither-painful-nor-pleasant feeling=> in the fourth jhāna neither-painful-nor-pleasant feeling must arise].
Upekkhāsampayuttattāyeva cettha pītipi parihāyatīti.
and here happiness vanishes simply owing to their association with equanimity.
Sayādaw U Sīlānanda: The word 'equanimity' means neutral feeling.