The solution is executed with error 'out of bounds' on the line 7

I have received this bound error though the sample input and output match. I tried several ways to solve this error, but I couldn’t. Please help me to overcome this problem. And also please, explain why what is the main reason for this error.
My Code:

#include <iostream>
using namespace std;

int main(){
    int a[4];
    for(int i=1; i<=4; i++){
        cin >> a[i];
    }
    string s;
    cin >> s;

    int sum = 0;
    for(int i =0; i<s.size(); i++){
        if(s[i]=='1'){
            sum=sum+a[1];
        }
        else if(s[i]=='2'){
            sum+=a[2];
        }
        else if(s[i]=='3'){
            sum+=a[3];
        }
        else if(s[i]=='4'){
            sum+=a[4];
        }
    }
    cout << sum << endl;
}

Sample input:

1 2 3 4
123214

Output:

13

>Solution :

First of all, this is not correct

int a[4];
for(int i=1; i<=4; i++){
    cin >> a[i];
}

arrays in C++ are indexed from 0, so it should be if you want to have a[1] = 1

int a[5];
for(int i = 0; i < 5; i++){
    cin >> a[i];
}

Side note. You dont need the "look-up array". To sum numbers, you can just do:

sum += (s[i] - '0');