this is a great question - i have wondered the same, and not been able to find a satisfactory answer.
i don’t know pali, but found a few translations for consideration. looking ajahn sujato’s wonderful side-by-side english-pali comparison of the sutta:
for loving kindness, the heart’s release is “said to be a foreign kind of beauty”.
for the other three brahmaviharas an alternative translation is here:
this suggests for compassion, “having completely gone beyond perception of forms, the perception of repulsion having disappeared, not attending to the perception of diversity”, the result is infinite space.
for altruistic joy, “having gone beyond infinite space”, the result is infinite consciousness.
for equanimity, “having gone beyond infinite consciousness, there is no-thing”, resulting in nothing-ness.
notably the refrain of the practice for each of the brahmaviharas is the same:
It’s when a mendicant develops the heart’s release by [metta/karuna/mudita/uppekha] together with the awakening factors of mindfulness, investigation of principles, energy, rapture, tranquility, immersion,
and equanimity, which rely on seclusion, fading away, and cessation, and ripen as letting go
If they wish: ‘May I meditate perceiving the repulsive in the unrepulsive,’ that’s what they do.
If they wish: ‘May I meditate perceiving the unrepulsive in the repulsive,’ that’s what they do.
If they wish: ‘May I meditate perceiving the repulsive in the unrepulsive and the repulsive,’ that’s what they do.
If they wish: ‘May I meditate perceiving the unrepulsive in the repulsive and the unrepulsive,’ that’s what they do.
If they wish: ‘May I meditate staying equanimous, mindful and aware, rejecting both the repulsive and the unrepulsive,’ that’s what they do.
it seems it’s a progression then - from this basic practice, start with compassion and let go of perception of forms, repulsion, and diversity / difference, and infinite space results. then, with altruistic joy, go beyond this sense of infinite space and infinite consciousness results. then, with equanimity, go beyond this sense of infinite consciousness, and nothing results.
it seems there’s much more to this that i have not considered before.