What fear is Keats expressing in lines 1-4 of "When I Have Fears That May Cease to Be"?
Like the other responses state, the speaker is afraid of his own impending mortality. The poem is a meditation of the speaker’s feelings about death.
In the first four lines specifically, the speaker is referring to his vocation. This is where it is tempting to conflate the poet with the speaker, because Keats uses words like “pen” and “books” that are associated with writers. However, this could be instead interpreted as an artist’s fear: that he or she may not be able to express all they need to before his or her time on earth ends. The speaker’s “teeming brain” has so many ideas and emotions that he worries it is impossible to get them all out before the end of his life.
He could also be describing the “high-piled” library full of “ripened grain” as all the art he wants to consume before his death. Besides just writing, the speaker appreciates the work of others, and he expresses anxiety that he might not get to experience all of the great literature he wants to before his death.
Death, love and ambition are the three-fold themes of this poem. In the first four lines, the poet fears that he may die before he can achieve fame, or consummate his love. These lines show that he has rich and fertile imagination. This is brought about by the use of words like "glean'd," "garners," "full ripen'd grain." A harvest is the metaphor of fulfilment and Keats' mind yearns for it. His mind is full of thoughts and ideas but he has not reaped it all in the shape of poems. The reason is obvious. Death looms large over him and he fears that he may neither achieve fame nor consummate his love before death.
Death. More specifically, in those first four lines, Keats is afraid that he will die before he gets a chance to write all the ideas that are piled up in his brain. He's an ambitious artist, and he wants to achieve those ambitions before he dies.