If you want to use parameters for a,b,c,d in Laravel 4
Model::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);
});