How to make Custom Validation rule for group unique values in laravel

I have working on Laravel 5.6. I have 4 dependent unique columns but don't know how to validate these dependent columns here is my migration for unique constrain:

$table->unique(array('lvl4','document_type','nature_id','type_id'),'u_coa_lvl4_asn_dnt_uk');

lvl4 is array.

Thanks in advance

via Chebli Mohamed