Upekkha100 asked: “So one can get to jhana via focusing on one of these four: metta/karuna/mudita/upekkha?”
One gets to jhana by overcoming the kama loka.
Anariya jhana are attained by SUPPRESSING kama raga. Ariya jhana are attained by REMOVING kama raga, i.e., one would an Anagami to get to even to the first Ariya jhana.
There are two brahma realms below the one that can be reached via doing the metta bhavana:
“31 Realms of Existence”
So, it is a gradual process to get to higher brahma realms.