**256**.

The first number is the value for 1^1.

The second number is the value for 2^2.

The third number is the value for 3^3.

And the fifth number is the value for 5^5.

So the fourth number must be the value for 4^4, which is 256.

**256**.

The first number is the value for 1^1.

The second number is the value for 2^2.

The third number is the value for 3^3.

And the fifth number is the value for 5^5.

So the fourth number must be the value for 4^4, which is 256.

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