invalid conversion from const char* to char

#include <iostream>

using namespace std;
char convert(int,char,int*);
int main()
{   int n,m2;
    char b[4];
    cout << "Please input Julian:";
    cin >> n;
    convert(n,b[4],&m2);
    cout << b << m2;
    return 0;
}
char convert(int n,char b[4],int *m2)
{
    int a;
    if (n>360)
   

     **{b[4]="NUL";}**
        else {a=n/30;*m2=n%30;
    switch (a)
    {
        **case 0:b[4]="Jan";
        case 1:b[4]="Feb";
        case 2:b[4]="Mar";
        case 3:b[4]="Apr";
        case 4:b[4]="May";
        case 5:b[4]="Jun";
        case 6:b[4]="Jul";
        case 7:b[4]="Aug";
        case 8:b[4]="Sep";
        case 9:b[4]="Oct";
        case 10:b[4]="Nov";
        case 11:b[4]="Dec";**
    }}
return 0;}

加了引号的部分报错是为什么呀
该怎么解决呢
我需要利用该函数将Julian历法转换为公历如：输入5 输出 Jan 5

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c++求助invalid conversion from const char* to char