Determine all polynomials $p$ with real coefficients for which $p(0)=0$ and
$$f(f(n))+n=4f(n)\qquad\text{for all }n\in\mathbb N,$$where $f(n)=\lfloor p(n)\rfloor$.
1. We start with the given conditions:
   - \( p(0) = 0 \)
   - \( f(f(n)) + n = 4f(n) \) for all \( n \in \mathbb{N} \), where \( f(n) = \lfloor p(n) \rfloor \).

2. Assume \( p(x) \) is a polynomial of degree \( d \). Since \( p(0) = 0 \), we can write \( p(x) \) as:
   \[
   p(x) = a_d x^d + a_{d-1} x^{d-1} + \cdots + a_1 x
   \]
   where \( a_d \neq 0 \).

3. Consider the behavior of \( f(f(n)) \) and \( 4f(n) - n \) for large \( n \). Since \( f(n) = \lfloor p(n) \rfloor \), we have:
   \[
   f(n) \approx p(n) \quad \text{for large } n.
   \]

4. Substituting \( f(n) \approx p(n) \) into the functional equation, we get:
   \[
   f(f(n)) + n \approx 4f(n).
   \]

5. Since \( f(n) \approx p(n) \), we have:
   \[
   f(f(n)) \approx f(p(n)) \approx p(p(n)).
   \]

6. Therefore, the equation becomes:
   \[
   p(p(n)) + n \approx 4p(n).
   \]

7. For large \( n \), the dominant term in \( p(p(n)) \) will be \( a_d^2 n^{d^2} \). Thus, we have:
   \[
   a_d^2 n^{d^2} + n \approx 4a_d n^d.
   \]

8. For the degrees to match, we must have \( d^2 = d \), which implies \( d = 1 \) or \( d = 0 \). Since \( d = 0 \) would make \( p(x) \) a constant polynomial, which does not satisfy the given conditions, we must have \( d = 1 \).

9. Therefore, \( p(x) \) is a linear polynomial. Let:
   \[
   p(x) = ax.
   \]

10. Substituting \( p(x) = ax \) into the functional equation, we get:
    \[
    \lfloor a \lfloor an \rfloor \rfloor + n = 4 \lfloor an \rfloor.
    \]

11. For large \( n \), \( \lfloor an \rfloor \approx an \). Thus, the equation becomes:
    \[
    \lfloor a(an) \rfloor + n \approx 4an.
    \]

12. Simplifying, we get:
    \[
    a^2 n + n \approx 4an.
    \]

13. For the coefficients of \( n \) to match, we must have:
    \[
    a^2 + 1 = 4a.
    \]

14. Solving the quadratic equation \( a^2 - 4a + 1 = 0 \), we get:
    \[
    a = 2 \pm \sqrt{3}.
    \]

15. We need to check which of these values of \( a \) satisfies the original functional equation. 

16. For \( a = 2 - \sqrt{3} \):
    \[
    \lfloor (2 - \sqrt{3}) \lfloor (2 - \sqrt{3}) n \rfloor \rfloor + n \neq 4 \lfloor (2 - \sqrt{3}) n \rfloor.
    \]
    This does not fit.

17. For \( a = 2 + \sqrt{3} \):
    \[
    \lfloor (2 + \sqrt{3}) \lfloor (2 + \sqrt{3}) n \rfloor \rfloor + n = 4 \lfloor (2 + \sqrt{3}) n \rfloor.
    \]
    This fits.

Therefore, the polynomial \( p(x) = (2 + \sqrt{3}) x \) satisfies the given conditions.

The final answer is \( \boxed{p(x) = (2 + \sqrt{3}) x} \).