3 Let nothing be done through selfish ambition or conceit, but in lowliness of mind let each esteem others better than himself.
4 Let each of you look out not only for his own interests, but also for the interests of others.
The Law instructed men to love others as themselves. Now we are to consider others as betters.
selfish ambition (v.3) = faction, ambition, self-seeking — anything involving self-will — probably a reference to the troubles in the Philippian church
conceit (v.3) = empty glory, empty opinion, empty pride — any glory centered on ourselves. Our glory is to be in Christ (3:3)
lowliness of mind (v.3) — humility
esteem (v.3) — consider — not based on feelings but on external facts
better (v.3) — to excel or surpass
look (v.4) = regard as a goal — have a desire for, take an interest in
We are to act for the welfare of others always — not with no consideration of our own welfare (it says “also”), but never for our own welfare selfishly.