upekkha in 3rd and 4th jhana, as part of 7sb, and as part of 4bv

Upekkha in the 4th jhana, and upekkha-sambojjhanga (7th awakening factor) are the same. 
upekkha is more properly translated as equanimous-observation. 
upekkha = upa + ikkhati. The ikkhati is looking upon. 
Along with S&S, Upekkha does vipassana while one is in the 4 jhanas. 

With the 4th brahma vihara, there seems to be some connection, at least in quality of samadhi, with 4th jhana. 
But one key difference, 4th jhana as a samma samadhi would be directed for the purpose of realizing nirvana.
Whereas upekkha of 4bv for a non-buddhist, would just lead to brahma realm rebirth. 

Upekkha in the Jhānas
AN 5.28 After 4 jhana smiles, (5. Reviewing-sign)
AN 5.28 Reviewing-sign
AN 5.26: uses samadhi nimitta from AN 5.28 to enter jhāna and onwards
AN 7.38 seems to overlap with AN 46.2 dhamma-vicaya-bojjhanga
AN 7.39 same as AN 7.38
AN 7.61 Moggallana fighting drowsiness
(2. Recall dhamma using V&V, thinking and evaluation, and upekkha)
Upekkha as one of the 4 bv (brahma vihāra)
AN 6.13 upekkha is escape from rāga
Upekkha in the Jhānas
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Upekkha appears explicitly in 3rd and 4th jhāna. For the first two jhānas, perhaps the predominance of pīti and sukha as a vedana are too dominant for upekkha to be a factor.
The traditional interpretation of upekkha is one of passive equanimity, but a number of passages seems to suggest a more investigative, insightful function.
AN 3.101 balancing viriya, samadhi, and upekkha
An 7.61 working with S&S in vipassana role
SN 46.3 as upekkha-sambojjhanga, “looking upon the concentrated mind”