Why Buddha lined up four brahmavihara in the chronological order 'metta, karuna, mudita and upeksha'

The practitioner wishes to let the being took the benefit&happiness, lovingkindness, so the practitioner wishes to help that being when that being is suffering, wishes to congratulate with that being when he is happy, and is equanimity when whatever happen.

Upekka, equanimity, is not easier than the other three. The practitioner is ignoring, doing not care, when he practices Upekkha without practicing the other three before, i.e. the healthy practitioner is not equanimity but he is ignoring when he doesn't care the standing old woman on the bus while he is sitting.

See the path of purification brahmavihāraniddesa:

1. Ñ(IX,109): One who wants to develop these four should practice them towards beings first as the promotion of the aspect of benefit—and lovingkindness has the promotion of the aspect of benefit as its characteristic;
2. Ñ: and next, on seeing or hearing or judging that beings whose benefit has been thus wished for are at the mercy of suffering, they should be practiced as the promotion of the aspect of the removal of suffering—and compassion has the promotion of the aspect of the removal of suffering as its characteristic;
3. Ñ: and then, on seeing the success of those whose benefit has been wished for and the removal of whose suffering has been wished for, they should be practiced as being glad—and gladness has the act of gladdening as its characteristic;
4. Ñ: but after that, there is nothing to be done and so they should be practiced as the neutral aspect, in other words, the state of an onlooker—and equanimity has the promotion of the aspect of neutrality as its characteristic;

Ñ: therefore, since their respective aims are the aspect of benefit, etc., their order should be understood to correspond, with loving kindness stated first, then compassion, gladness, and equanimity.