sometimes-test.php

<?php

$data = [
	'a2' => null,
	'a3' => '',
	'a4' => [],
	'b2' => null,
	'b3' => '',
	'b4' => [],
	'c2' => null,
	'c3' => '',
	'c4' => [],
	'd2' => null,
	'd3' => '',
	'd4' => [],
];
$rules = [
	'a1' => 'sometimes|required|min:10',
	'a2' => 'sometimes|required|min:10',
	'a3' => 'sometimes|required|min:10',
	'a4' => 'sometimes|required|min:10',
	'b1' => 'filled|min:10',
	'b2' => 'filled|min:10',
	'b3' => 'filled|min:10',
	'b4' => 'filled|min:10',
	'c1' => 'min:10',
	'c2' => 'min:10',
	'c3' => 'min:10',
	'c4' => 'min:10',
	'd1' => 'required|min:10',
	'd2' => 'required|min:10',
	'd3' => 'required|min:10',
	'd4' => 'required|min:10',
];

Validator::make($data, $rules)->errors();

// The a2 field is required.
// The a3 field is required.
// The a4 field is required.
// The b2 field is required.
// The b3 field is required.
// The b4 field is required.
// The c2 must be at least 10 characters.
// The c4 must be at least 10 characters.
// The d1 field is required.
// The d2 field is required.
// The d3 field is required.
// The d4 field is required.