4-6

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    FILE*in = fopen("in.txt","r");
    FILE*out = fopen("out.txt","w");
    char c[256];
    fgets(c,256,in);
    for(int i=0; i<strlen(c); i++){
        if(c[i]>='0' && c[i]<='9'){
            c[i]='_';}
    }
    fprintf(out, "%s",c);

    return 0;
}