...$y >> a$¹

Hint: since the net charge balances (=0), we expect no monopolar part (like $1/x^2$). Since the dipoles do not quite balance, we might see a dipolar part (like $1/x^3$). However, since the dipoles are not parallel we might expect to see a significant quadrupolar term that varies like $1/x^4$ as well.

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...$y >> a$²

Hint: since the net charge balances (=0), we expect no monopolar part (like $1/x^2$). Since the dipoles do not quite balance, we might see a dipolar part (like $1/x^3$). However, since the dipoles are not parallel we might expect to see a significant quadrupolar term that varies like $1/x^4$ as well.

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