In this canto of "Ode to the West Wind," Shelley supposes that if he were like a leaf, cloud, or wave, he too could take part in the regenerative processes of death and life in nature. In the first canto of the poem, Shelley refers to the wind as "Destroyer and Preserver." The wind in autumn signals death; the leaves and seeds blown away to lay dormant until spring when they will emerge as new life.
In the fourth canto, Shelley is comparing his poetry in particular with this idea of being sent (via the metaphoric wind) all over the world. The idea is that his poetry could serve humankind as a kind of regenerative power; something that will implore others to consider the regenerative power in nature and in themselves.
Shelley personifies the wind as if it were a god or, in terms of humanity, the mover of history. Shelley wants to be part of this movement and wishes to influence others even after his death (which is to be compared with the dying leaves in autumn).
This regenerative process in nature is to be compared with the actual creation of poetry as well as the proliferation of it (spreading a message to others via the metaphorical power of the wind).
The impulse of thy strength, only less free
Than thou, O Uncontrollable! If even
I were as in my boyhood, and could be
Thy comrade of thy wandering over Heaven, (46-49)
There is also a sense that Shelley looks to the wind as a mentor; someone/something to guide him. Thus, he in his "boyhood" might travel as the comrade of the wind. And in a more spiritual or creative (in terms of the poetic imagination) sense, Shelley is making a prayer to the wind (as an inspiring regenerative power) to lift him, make him more free to create and be heard/read. "A heavy weight of hours has chained and bowed / One too like thee: tameless, and swift, and proud." A more broad interpretation of these last lines is this: Shelley wishes to be free of mental and physical constraints so that his creative powers may flourish and poetry will continue to inspire.