$a, $b, $c, $d can be dynamic values by the query
->where(function($query) use ($a, $b)
{
$query->where('a', $a)
->orWhere('b',$b);
})
->where(function($query) use ($c, $d)
{
$query->where('c', $c)
->orWhere('d',$d);
})