Let $a_n$ be the number of sets $S$ of positive integers for which
\[ \sum_{k\in S}F_k=n,\]
where the Fibonacci sequence $(F_k)_{k\ge 1}$ satisfies $F_{k+2}=F_{k+1}+F_k$ and begins $F_1=1$, $F_2=1$, $F_3=2$, $F_4=3$. Find the largest number $n$ such that $a_n=2020$.
To solve the problem, we need to find the largest number \( n \) such that the number of sets \( S \) of positive integers for which the sum of the Fibonacci numbers indexed by \( S \) equals \( n \) is exactly 2020. 

1. **Understanding Zeckendorf's Representation**:
   Zeckendorf's theorem states that every positive integer can be uniquely represented as the sum of one or more distinct, non-consecutive Fibonacci numbers. This representation is called the Zeckendorf representation.

2. **Binary Representation**:
   We can associate each set \( S \) with a binary string where the \( k \)-th bit is 1 if \( F_k \) is included in the sum and 0 otherwise. For example, the set \( \{F_4, F_3, F_1\} \) corresponds to the binary string \( 1101 \).

3. **Counting Valid Representations**:
   The problem requires us to count the number of valid binary strings (Zeckendorf representations) that sum to \( n \). Each valid binary string must not have consecutive 1s, as this would violate the non-consecutive condition of Zeckendorf's theorem.

4. **Recursive Formula**:
   Let \( a_n \) be the number of valid sets \( S \) such that the sum of the Fibonacci numbers indexed by \( S \) equals \( n \). We need to find the largest \( n \) such that \( a_n = 2020 \).

5. **Generating Function Approach**:
   The number of ways to write \( n \) as a sum of distinct, non-consecutive Fibonacci numbers can be found using a generating function approach. The generating function for the Fibonacci sequence is:
   \[
   G(x) = \sum_{k=0}^{\infty} F_k x^k = \frac{x}{1 - x - x^2}
   \]
   However, we need to count the number of ways to form \( n \) using distinct Fibonacci numbers, which is more complex.

6. **Binary String Length**:
   The length of the binary string corresponding to the Zeckendorf representation of \( n \) is related to the Fibonacci numbers. The maximal length of such a string with \( 2020 \) valid representations is \( 2 \times 2020 - 1 = 4039 \).

7. **Largest \( n \)**:
   The largest \( n \) such that \( a_n = 2020 \) corresponds to the sum of the first 2020 Fibonacci numbers. The sum of the first \( k \) Fibonacci numbers is given by:
   \[
   F_1 + F_2 + \cdots + F_k = F_{k+2} - 1
   \]
   Therefore, for \( k = 2020 \):
   \[
   n = F_{2022} - 1
   \]

The final answer is \( \boxed{F_{2022} - 1} \).