The fourth stanza of this poem shows the speaker away from the daffodils, at home, and thinking back upon them.  The thought is demonstrated as a picture in his "inner eye."  This picture brings him joy and he "dances with the daffodils."  The rejuvenating spirit of nature is highlighted in this stanza, and Wordsworth uses personification and metaphor in this stanza for his meaning.

First he personifies the daffodils, giving them the power to "flash upon" his inner eye.  This echoes the personification of the earlier stanzas, when Wordsworth brings the daffodils to life in their dancing ways.  The reason for this is to honor this aspect of nature in a humanly way, giving it more importance and power. 

Metaphor is used in the last two lines, when the speaker suggests that he "dances" with them.  He is comparing his uplifted spirits with a literal dance in order to demonstrate how effective the image of nature is in uplifting his spirits.