The poem opens with the use of apostrophe. By having the speaker address Prometheus directly, Byron is able to immediately create a sense of closeness between the speaker and Prometheus; though Prometheus is not present, the speaker (and reader) feels an affinity with him.
The apostrophe also emphasizes that Prometheus is not a human being. Instead, he is one of the Titans—in Greek mythology, a race of gods who ruled before the Olympians (Zeus, Hera, Aphrodite, etc.) took over. The reader is thus introduced to a significant aspect of Prometheus's character within the first four lines. If Prometheus is a god, and gods do not despise human suffering, then why is Prometheus moved to help humans? Prometheus is at odds with his own kind. There is a dissonance between who he is expected to be and who he actually is.
Thematically, this dissonance upsets the status quo—the status quo being like an equation where on one side there is human suffering, and on the other the gods looking on, unbothered. By caring about the suffering of humans, Prometheus has upset this equation, unbalancing the very reality of the world. Things that were taken for granted as true before may no longer be treated as such.
It's subtle, but the first four lines both establish and undermine the rules of the poem. For example, simply by addressing Prometheus, the speaker has disrupted the norm of the poem's meter: the trochee created by the word "Titan" at the beginning of the poem is in contrast to the iambic tetrameter seen throughout the rest of these four lines (recall that an iamb is a poetic foot with a da DUM rhythm, and tetrameter means there are four of those feet per line).
This happens at the very beginning of the poem, which thrusts the reader into a narrative where Prometheus's rebellion has already occurred. Indeed, the story of Prometheus is being told in past tense; the gods' tyrannical reign has already been disrupted. The speaker is recalling it now because it bears significance to what the poem hopes to convey. The reader is made (perhaps subconsciously) aware that the speaker addresses Prometheus as a stand-in for any individual who rebels against oppressive norms.
Within the first four lines, the poem also establishes an obvious rhythm. The clear, full end rhymes form a rhyme pattern of ABBA (known as an enclosed rhyme). There is a mirror-like quality to this initial rhyme pattern, the reflection of AB being BA. This is notable as the lines seem to mirror each other thematically: the first line addresses Prometheus's "immortal eyes," the second what he is seeing (human suffering); the third line again addresses what he is seeing (the sad reality of humanity), and the fourth again addresses who is seeing this scene, though this time more generally.
This is important as the poem is essentially using Prometheus as a mirror for the "godlike" aspects of the speaker. In summoning the spirit of Prometheus, the speaker hopes to speak to his own heroic potential, and to that of the reader as well.