Find all pairs $(m, n)$ of positive integers for which $4 (mn +1)$ is divisible by $(m + n)^2$.
To find all pairs \((m, n)\) of positive integers for which \(4(mn + 1)\) is divisible by \((m + n)^2\), we start by setting up the condition:

\[
\frac{4(mn + 1)}{(m + n)^2} \in \mathbb{Z}
\]

This implies that \((m + n)^2\) must divide \(4(mn + 1)\). Let's analyze this condition step by step.

1. **Initial Inequality Analysis:**
   We know that for any positive integers \(m\) and \(n\), the following inequality holds:
   \[
   (m + n)^2 \leq 4(mn + 1)
   \]
   Expanding and rearranging terms, we get:
   \[
   m^2 + 2mn + n^2 \leq 4mn + 4
   \]
   Simplifying further:
   \[
   m^2 + n^2 - 2mn \leq 4
   \]
   \[
   (m - n)^2 \leq 4
   \]

2. **Possible Values for \((m - n)^2\):**
   Since \((m - n)^2\) is a perfect square and must be less than or equal to 4, the possible values are:
   \[
   (m - n)^2 = 0, 1, 4
   \]
   This gives us three cases to consider:
   - \(m - n = 0\)
   - \(m - n = \pm 1\)
   - \(m - n = \pm 2\)

3. **Case Analysis:**
   - **Case 1: \(m = n\)**
     \[
     (m - n)^2 = 0 \implies m = n
     \]
     Substituting \(m = n\) into the original condition:
     \[
     4(mn + 1) = 4(m^2 + 1)
     \]
     \[
     (m + n)^2 = (2m)^2 = 4m^2
     \]
     Clearly, \(4(m^2 + 1)\) is divisible by \(4m^2\), so \((m, n) = (k, k)\) for any positive integer \(k\).

   - **Case 2: \(m - n = \pm 1\)**
     \[
     (m - n)^2 = 1 \implies m = n + 1 \text{ or } m = n - 1
     \]
     Without loss of generality, assume \(m = n + 1\):
     \[
     4((n + 1)n + 1) = 4(n^2 + n + 1)
     \]
     \[
     (m + n)^2 = (n + 1 + n)^2 = (2n + 1)^2 = 4n^2 + 4n + 1
     \]
     We need:
     \[
     4(n^2 + n + 1) \text{ to be divisible by } 4n^2 + 4n + 1
     \]
     This is not generally true for all \(n\), so no solutions in this case.

   - **Case 3: \(m - n = \pm 2\)**
     \[
     (m - n)^2 = 4 \implies m = n + 2 \text{ or } m = n - 2
     \]
     Without loss of generality, assume \(m = n + 2\):
     \[
     4((n + 2)n + 1) = 4(n^2 + 2n + 1)
     \]
     \[
     (m + n)^2 = (n + 2 + n)^2 = (2n + 2)^2 = 4n^2 + 8n + 4
     \]
     We need:
     \[
     4(n^2 + 2n + 1) \text{ to be divisible by } 4n^2 + 8n + 4
     \]
     This is not generally true for all \(n\), so no solutions in this case.

Therefore, the only solutions are when \(m = n\).

The final answer is \( \boxed{ (m, n) = (k, k) } \) for any positive integer \(k\).