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You can simply do this by creating a big image which contain all your input image, then copy your input images to the appropriate location of the created image using opencv setROI and resetROI and finally display the destination image . The below code assumes that all the images are with same height and width.

And also it is always recommended to use OpenCV C++ API instead of old C interface, for more info see this links, link1, link2

#include <opencv2/imgproc/imgproc.hpp>  
#include <opencv2/core/core.hpp>        
#include <opencv2/highgui/highgui.hpp>
#include <iostream>


using namespace cv;
using namespace std;

int main()
{
//Image Reading
IplImage* img1 = cvLoadImage( "ball.jpg" );
IplImage* img2 = cvLoadImage( "ball.jpg" );
IplImage* img3 = cvLoadImage( "ball.jpg" );
IplImage* img4 = cvLoadImage( "ball.jpg" );

int dstWidth=img1->width+img1->width;
int dstHeight=img1->height+img1->height;

IplImage* dst=cvCreateImage(cvSize(dstWidth,dstHeight),IPL_DEPTH_8U,3); 

// Copy first image to dst
cvSetImageROI(dst, cvRect(0, 0,img1->width,img1->height) );
cvCopy(img1,dst,NULL);
cvResetImageROI(dst);

// Copy second image to dst
cvSetImageROI(dst, cvRect(img2->width, 0,img2->width,img2->height) );
cvCopy(img2,dst,NULL);
cvResetImageROI(dst);

// Copy third image to dst
cvSetImageROI(dst, cvRect(0, img3->height,img3->width,img3->height) );
cvCopy(img3,dst,NULL);
cvResetImageROI(dst);

// Copy fourth image to dst
cvSetImageROI(dst, cvRect(img4->width, img4->height,img4->width,img4->height) );
cvCopy(img4,dst,NULL);
cvResetImageROI(dst);

//show all in a single window
cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZE );
cvShowImage( "Example1", dst );
cvWaitKey(0);

}