Find all positive integers $n$ such that $2021^n$ can be expressed in the form $x^4-4y^4$ for some integers $x,y$.
1. We start by considering the equation \(2021^n = x^4 - 4y^4\) for some integers \(x\) and \(y\). We need to find all positive integers \(n\) that satisfy this equation.

2. First, we consider the cases where \(x = 0\) or \(y = 0\):
   - If \(x = 0\), then \(2021^n = -4y^4\), which is not possible since \(2021^n\) is positive.
   - If \(y = 0\), then \(2021^n = x^4\). This implies \(2021^n\) must be a perfect fourth power. Therefore, \(n\) must be a multiple of 4, i.e., \(n \equiv 0 \pmod{4}\).

3. Now, suppose \(x \neq 0\) and \(y \neq 0\). Let \(\gcd(x, y) = d\) and write \(x = da\) and \(y = db\) where \(\gcd(a, b) = 1\). Substituting these into the equation, we get:
   \[
   2021^n = (da)^4 - 4(db)^4 = d^4(a^4 - 4b^4)
   \]
   This implies:
   \[
   2021^n = d^4(a^4 - 4b^4)
   \]
   Since \(2021 = 43 \times 47\), we can write:
   \[
   2021^n = 43^n \times 47^n
   \]
   Therefore:
   \[
   d^4(a^4 - 4b^4) = 43^n \times 47^n
   \]

4. Let \(d = 43^k \times 47^l\) where \(0 \leq k, l \leq \frac{n}{4}\). Then:
   \[
   (a^4 - 4b^4) = 43^{n-4k} \times 47^{n-4l}
   \]

5. **Claim 1:** \(\gcd(a^2 - 2b^2, a^2 + 2b^2) = 1\)

   *Proof:* Assume there is a prime \(p\) such that \(p \mid a^2 - 2b^2\) and \(p \mid a^2 + 2b^2\). Then:
   \[
   p \mid (a^2 - 2b^2) + (a^2 + 2b^2) \implies p \mid 2a^2
   \]
   If \(p = 2\), then \(2 \mid a^2 - 2b^2\) implies \(2 \mid a\), which is a contradiction since \(a\) and \(b\) are coprime. Hence, \(p \mid a\). Similarly:
   \[
   p \mid (a^2 + 2b^2) - a^2 \implies p \mid 2b^2 \implies p \mid b
   \]
   Thus, \(\gcd(a, b) \geq p\), which contradicts \(\gcd(a, b) = 1\). Therefore, \(\gcd(a^2 - 2b^2, a^2 + 2b^2) = 1\).

6. **Claim 2:** \(47 \nmid a^2 + 2b^2\)

   *Proof:* Assume \(47 \mid a^2 + 2b^2\). Since \(\gcd(a, b) = 1\), we have \(\gcd(a, 47) = \gcd(b, 47) = 1\). Then:
   \[
   a^2 + 2b^2 \equiv 0 \pmod{47} \implies \left(\frac{a}{b}\right)^2 \equiv -2 \pmod{47}
   \]
   Using the Legendre symbol:
   \[
   \left(\frac{-2}{47}\right) = \left(\frac{-1}{47}\right) \left(\frac{2}{47}\right)
   \]
   Since \(\left(\frac{-1}{47}\right) = (-1)^{\frac{47-1}{2}} = -1\) and \(\left(\frac{2}{47}\right) = (-1)^{\frac{47^2-1}{8}} = 1\), we get:
   \[
   \left(\frac{-2}{47}\right) = -1
   \]
   This implies \(-1 = 1\), a contradiction. Hence, \(47 \nmid a^2 + 2b^2\).

7. Combining these claims, we have:
   \[
   a^2 - 2b^2 = 47^{n-4l} \quad \text{and} \quad a^2 + 2b^2 = 43^{n-4k}
   \]
   Since \(a^2 + 2b^2 > a^2 - 2b^2\), we get \(n - 4k > 0\). Thus:
   \[
   4b^2 = (a^2 + 2b^2) - (a^2 - 2b^2) = 43^{n-4k} - 47^{n-4l}
   \]
   So \(43^r - 47^s\) is a perfect square for some integers \(r > 0\) and \(s \geq 0\). Taking modulo 43 gives:
   \[
   \left(\frac{43^r - 47^s}{43}\right) = 1 \implies \left(\frac{-4^s}{43}\right) = 1 \implies \left(\frac{-1}{43}\right) \left(\frac{4^s}{43}\right) = 1
   \]
   Since \(\left(\frac{-1}{43}\right) = -1\) and \(\left(\frac{4^s}{43}\right) = 1\), we get:
   \[
   (-1) \cdot 1 = 1
   \]
   This is a contradiction. Hence, the only solution is \(n = 4m\) where \(m\) is any positive integer.

The final answer is \( \boxed{ n = 4m } \) where \(m\) is any positive integer.