Segmentation fault in openMP program with SSE instructions with number of threads bigger than 4

I wrote a simple c++ openMP program that use SSE instructions, and I am facing the Segmentation fault when the number of threads is bigger than 4. I am using g++ as compiler and linux as OS.

Please help me

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <emmintrin.h>
#include <assert.h>
#include <stdint.h>
#include <omp.h>

unsigned **a;

void f(int input_index){
    int j;
    __m128i* t = (__m128i*) a[input_index];
    for(j=0; j<4; j++)
            t[j] = _mm_set1_epi32(input_index* lenS);
}

int main(int argc, char *argv[])
{
int i,j,nThreads,tid;
bitD = new unsigned*[4];
for(i=0; i<4; i++)
    bitD[i] = new unsigned[16];

omp_set_num_threads(8); 

#pragma omp parallel private(i,nThreads,tid)
{
    tid = omp_get_thread_num();
    nThreads = omp_get_num_threads();                
    for(i=0; i<(4/nThreads); i++){
            f(i*nThreads+tid);                        
     }              
}


for(i=0; i<4; i++)
    for(j=0; j<16; j++)
        printf("a[%d][%d]=%d\n",i,j,bitD[i][j]);
}