You might could do something like this (php must be enabled on input)
$entries = array();
$sql = "SELECT author_id as author,
max(entry_id) as entry_id
WHERE channel_id = '4'
AND status = 'open'
GROUP BY author
ORDER BY entry_id DESC
Stash itself doesn't have modifiers for variables, such as format="". It can only capture strings from other tags. You will need to save the formatted dates when you set the Stash list if you want to use them in a comparison when getting the list.