In "Death Be Not Proud", what do the first four lines mean?WHAT CAN YOU INFER ABOUT THE SPEAKER'S ATTITUDE ABOUT DEATH IN LINE EIGHT?

One might paraphrase the first four lines of Donne's "Death Be Not Proud" in the following way:

Don't be proud Death, for although some in the past have called you

mighty and dreadful [something to be dreaded], you are not;

for those you think you overcome [kill]

don't really die, you mistaken fool, and you can't kill me, either.

The "mistaken fool" reference might be a little strong, but that's the general idea.  Death is not powerful and is not to be feared, because those who die are not really dead. 

The speaker apostrophizes death, speaking to it as if it were human, and reveals in the remainder of the poem what is behind his thinking and his bold statements:  humans only sleep when they die, for they will "wake eternally" and death will exist no more--death shall be the one to die. 

Donne's apostrophe leads to his paradox--humans don't really die when they die and, in the end, death is what will die. 

Line eight simply contributes to the paradox:  when men die they are only resting, and will later awake to their soul's salvation.  The speaker's attitude, revealed in this line and the entire poem, is a bit sarcastic and flippant.  He is disrespectful toward Death, and is putting it into its proper place, as he sees it.