FILE *f1;
FILE *f2;
if((fold = fopen("a.txt","w"))==NULL)return 0;
if((fnew = fopen("b.txt","w"))==NULL)return 0;
for(int i=0;i<4;i++)
{
fgets(buf,256,f1);
fputs(buf,f2);
}
fclose(f1);
fclose(f2);

こんな感じなんですけど、どうでしょうか