The expression $(m+n)4$ can be simplified by distributing the 4 across the terms inside the parentheses:

$(m + n)4 = 4(m + n)$

Now distribute the 4 to both $m$ and $n$:

$4(m + n) = 4m + 4n$

So, the simplified expression is:

$4m + 4n$

Would you like further clarification or examples on this type of problem?

Here are some related questions:

1. How would you simplify $(x + y)5$?
2. What happens if you distribute a negative number, such as $(-2)(a + b)$?
3. Can you apply this method to polynomials of more than two terms?
4. How do you expand expressions like $(x + y + z)3$?
5. What are the properties of distribution in algebra?

Tip: Always remember that distributing a number to terms inside parentheses follows the distributive property, $a(b + c) = ab + ac$.