Find the coefficient of `x^4` in the expansion of `(1+x+x^2+x^3)^(6)dot`

Explanation

To find the coefficient of \( x^4 \) in the expansion of \( (1 + x + x^2 + x^3)^6 \), we can follow these steps:

### Step 1: Rewrite the expression
We can rewrite \( (1 + x + x^2 + x^3)^6 \) as \( (1 + x + x^2 + x^3)^6 \).

### Step 2: Use the multinomial theorem
The multinomial theorem states that:
\[
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