Go: package main

import "fmt"

func main() {
	var i int64
	var list = []int64{13, 19, 109, 199, 829, 1489, 1879, 2089, 3259, 3469, 5659, 9439, 13009, 15649, 15739, 16069, 18049, 18919, 19429, 21019, 22279, 25309, 31729, 34849, 43789, 51349, 55339, 62989, 67219, 69499, 72229, 77269, 79699, 81049, 82729, 88819, 97849, 99139, 101119, 109849, 116539, 119299, 122209, 135469, 144169, 157279, 165709, 166849, 171169, 187639, 194869, 195739, 201499, 201829, 217369, 225349, 240049, 243709, 247609, 247999, 257869, 260419, 266689, 268819, 276049, 284749, 285289, 294319, 295879, 299479, 300499, 301999, 326149, 334429, 340939, 346399, 347989, 354259, 358909, 361219, 375259, 388699, 389569, 392269, 394819, 397549, 397759, 402139, 402769, 412039, 419059, 420859, 427249, 442579, 444349, 452539, 463459, 465169, 467479, 470089, 477019, 490579, 495619, 500239, 510619, 518809, 536449, 536779, 539509, 549169, 559219, 563419, 570049, 572659, 585919, 594829, 597679, 607309, 622249, 626629, 632089, 632329, 633469, 633799, 654169, 657499, 661099, 663589, 664669, 666439, 680299, 681259, 691729, 705169, 715159, 734479, 736369, 739399, 768199, 773029, 795799, 803449, 814069, 822169, 823729, 829729, 833719, 837079, 845989, 854929, 855739, 857959, 875269, 876019, 881479, 889879, 907399, 930079, 938059, 946669, 954979, 958549, 959479, 976309, 978079, 983449, 1002349, 1003369, 1006309, 1006339, 1008859, 1015369, 1022389, 1022509, 1023949, 1027759, 1043599, 1063969, 1065019, 1068259, 1068709, 1091269, 1093069, 1093999, 1100839, 1117609, 1117819, 1118869, 1120549, 1121839, 1122139, 1126669, 1137889, 1146799, 1155619, 1156039, 1157839, 1163719, 1167709, 1168249, 1170139, 1172029, 1172539, 1173589, 1182289, 1210879, 1228399, 1230379, 1233439, 1246249, 1246369, 1257079, 1272289, 1285519, 1298119, 1322179, 1322599, 1324579, 1329709, 1337269, 1339909, 1340329, 1351249, 1352209, 1358809, 1360789, 1368469, 1381279, 1400809, 1402369, 1410979, 1440589, 1447009, 1451839, 1461409, 1468639, 1508629, 1514329, 1524079, 1525969, 1540969, 1571749, 1573549, 1573939, 1577299, 1584439, 1588759, 1592869, 1603339, 1615849, 1616809, 1621729, 1627609, 1631059, 1653109, 1659109, 1670569, 1678759, 1681879, 1685449, 1691869, 1711819, 1718869, 1727779, 1747729, 1748479, 1749499, 1755829, 1764229, 1780489, 1791739, 1797379, 1798639, 1802659, 1819849, 1830349, 1833529, 1834039, 1837399, 1857679, 1890529, 1902619, 1904479, 1910269, 1912069, 1912459, 1915939, 1917739, 1925389, 1943659, 1954159, 1954369, 1959319, 1979149, 1979899, 2002339, 2007619, 2007919, 2016409, 2020729, 2050339, 2062009, 2063779, 2065579, 2075839, 2079199, 2083519, 2084449, 2087389, 2107669, 2116579, 2136139, 2141809, 2143489, 2144509, 2157739, 2159239, 2168659, 2176639, 2177509, 2193889, 2202799, 2203969, 2223679, 2225059, 2246149, 2248249, 2256349, 2262979, 2266639, 2289649, 2290039, 2294059, 2303599, 2340259, 2342779, 2362279, 2373409, 2376169, 2381089, 2403889, 2413429, 2418679, 2423419, 2443789, 2458669, 2470339, 2478529, 2479669, 2499949, 2508049, 2535109, 2541949, 2546239, 2552119, 2552659, 2561269, 2564329, 2576599, 2594959, 2594989, 2596669, 2604739, 2614069, 2635489, 2668249, 2673529, 2674549, 2679499, 2683789, 2696929, 2704909, 2712379, 2715289, 2728549, 2731699, 2731909, 2759299, 2761729, 2764129, 2790259, 2805169, 2822719, 2839939, 2840269, 2846869, 2849689, 2875339, 2879299, 2893489, 2918569, 2927599, 2927809, 2952799, 2954689, 2989039, 2990839, 3047419, 3058879, 3062209, 3063499, 3066829, 3076399, 3089329, 3092569, 3101479, 3103279, 3129619, 3153589, 3157579, 3171739, 3187609, 3200209, 3207439, 3208819, 3215749, 3224869, 3243349, 3277699, 3295549, 3308959, 3326629, 3328909, 3340879, 3366829, 3371449, 3374479, 3378049, 3400219, 3416059, 3436249, 3437719, 3467539, 3479899, 3512059, 3512239, 3512989, 3513079, 3514549, 3518329, 3550699, 3584929, 3586909, 3593509, 3621469, 3652939, 3669769, 3690529, 3713449, 3727729, 3735169, 3741169, 3798079, 3817819, 3837139, 3837859, 3849829, 3854119, 3894049, 3919219, 3919

C++: 
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
#include <iomanip>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }


vector<bool> getPrimeTable(const int& max)
{
	vector<bool> table(max + 1);
    
	table[2] = true;
    
	int i, j;
	for(i = 3; i <= max; i += 2)
	{
		if(!(table[i] = i % 2 == 1)) continue;
		for(j = 3; j*j <= i; j += 2)
		{
			if(i % j == 0)
			{
				table[i] = false;
				break;
			}
		}
	}
    
	return table;
}

bool isPrime(int x)
{
	if(x == 2) return true;
	if(x % 2 == 0 || x < 2) return false;

	for(int i = 3; i*i <= x; i += 2)
	{
		if(x % i == 0)
		{
			return false;
		}
	}

	return true;
}

int main()
{
	// 0 : false, 1 : true, 2 : unknown
	vint table(10000000, 2);

	int n;
	while(cin >> n && n)
	{
		int dx[] = {0, -2, -6, -8};

		if(n % 2 == 0) n--;
		for(int i = n; 8 <= i; i -= 2)
		{
			rep(j, 4)
			{
				int nn = i + dx[j];
				
				if(!(table[nn] == 1 || (table[nn] == 2 && (table[nn] = isPrime(nn)))))
					break;
				else if(j == 3)
				{
					cout << i << endl;
					goto ng;
				}
			}
		}

ng:
		;
	}
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <set>

int n;
std::vector<bool> pr(10000001, true);
std::set<int> sets;

int main() {
    for(int i = 2; i <= 10000000; ++i) {
        if(pr[i]) {
            for(long long j = ((long long)i) * i; j <= 10000000ll; j += i) {
                pr[j] = false;

            }
            if(i >= 13) {
                if(pr[i - 2] && pr[i - 6] && pr[i - 8]) {
                    sets.insert(i);
                }
            }
        }
    }

    while(std::cin >> n, n) {
        auto it  = sets.upper_bound(n);
        --it;

        std::cout << *it << std::endl;
    }
}

C++: #include<iostream>
#include<vector>

using namespace std;

int main()
{
	int n;
	int i;
	vector<int> prime,prime2;
	vector<int> fourprime;
	
	for(i=0;i<1000000;i++)
	{
		prime.push_back(1);
	}
	
	prime[0]=prime[1]=0;
	
	for(i=0;i<1000000;i++)
	{
		if(prime[i]==1)
		{
			prime2.push_back(i);
			for(int j=2;j*i<1000000;j++)
			{
				prime[j*i]=0;
			}
		}
	}
	
	for(i=0;i<prime2.size();i++)
	{
		if(i+8<prime2.size())
		{
			if(prime2[i]+2==prime2[i+1] && prime2[i]+6==prime2[i+2] && prime2[i]+8==prime2[i+3])
			{
				fourprime.push_back(prime2[i+3]);
			}
		}
	}
	
	
	for(;;)
	{
		cin >> n ;
		if(n==0)
			break;
		if(fourprime[fourprime.size()-1]<=n)
			cout << fourprime[fourprime.size()-1] << endl;
		else
		{
			for(i=0;i<fourprime.size();++i)
			{
				if(fourprime[i]>n)
				{
					cout << fourprime[i-1] << endl;
					break;
				}
				if(fourprime[i]==n)
				{
					cout << fourprime[i] << endl;
					break;
				}
			}
		}
		
	}
}