str.resize(4);//Не обязательно так, но суть такова
stream.read(&str[0],4);

namespace use_fixed_str
{
    std::basic_istream<char>& operator>>( std::basic_istream<char>& stream, std::string& str )
    {
        for( std::string::iterator it = str.begin(); str.end() != it; ++ it )
        {
            stream >> *it;
        }
        return stream;
    }
};

....

std::string str;
std::cin >> str; // читаем всю строку
using namespace use_fixed_str;
str.resize( 4 );
std::cin >> str; // читаем по 4 символа
std::cin >> str;

L>str.resize(4);//Не обязательно так, но суть такова
L>stream.read(&str[0],4);
L>

template<class _Elem,
class _Traits,
class _Alloc> inline
    basic_istream<_Elem, _Traits>& __CLRCALL_OR_CDECL get_nchars_line(
    basic_istream<_Elem, _Traits>& _Istr,
    basic_string<_Elem, _Traits, _Alloc>& _Str,
    const size_t _Count)

21.3.7.9
1 Effects: Begins by constructing a sentry object k as if k were constructed by typename
basic_istream<charT,traits>::sentry k(is). If bool(k) is true, it calls
str.erase() and then extracts characters from is and appends them to str as if by calling
str.append(1,c). If is.width() is greater than zero, the maximum number n of characters
appended is is.width(); otherwise n is str.max_size(). Characters are extracted and
appended until any of the following occurs:
— n characters are stored;
— end-of-file occurs on the input sequence;
— isspace(c,is.getloc()) is true for the next available input character c.

#include <sstream>
#include <iostream>

using namespace std;

int main()
{
  stringstream ss;
  ss << "0123456789";

  string s;

  streamsize old = ss.width(4);
  ss >> s;
  cout << s;

  ss.width(old);
  ss >> s;

  cout << '\n' << s;
}

0123
456789

isspace(c,is.getloc()) is true for the next available input character c.