Filtering and sorting Laravel collection

I have this collection:

    '1234' => [
        [
            'name' => 'test',
            'date' => '2020-12-10'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-12-10'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-12-11'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-12-11'
        ]
    ],
    '4321' => [
        [
            'name' => 'test',
            'date' => '2020-03-04'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-03-04'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-08-06'
        ],
        [ 
            'name' => 'another test'
            'date' => '2020-08-06'
        ]
    ]
]

The 1234 and 4321 are IDs of a subscription. There's a lot of those.

The items inside are notifications sent for each of them. There could be 1 to 4 UNIQUE notifications and each one could repeat twice, so up to 8 records.

So it's like this

   Notification January
   Notification January
   Notification February
   Notification February
   Notification March
   Notification March

Now obviously those are timestamps.

I want to order each Subscription's notifications by date and then extract ALL first notifications in a $first collection, all second notifications in $second collection and so on up to fourth.

So in $first there should be the earliest sent notification for all subscriptions and any duplicates.

via Chebli Mohamed