Real numbers $a, b, c, d$ satisfy $a^2 + b^2 + c^2 + d^2 = 4$. Find the greatest possible value of $E(a, b, c, d) = a^4 + b^4 + c^4 + d^4 + 4(a + b + c + d)^2$.

Let's think step by step and output the final answer within \boxed{}.
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