Recent post from users

Hi guys!

I’ve got a requst from my client, “Can we show the 4 most recent post from any user?” – That was easy. Done and done.

Next day my client comes back to me and would like to make a minor fix:

4 lastest post from users. A user can only appear once( only one post from a user, but still 4 most recent).

I’ve been struggling with this a while no so i hope you can help me.
Using a plugin, sorta looks like this
$user = get_userdata(15);

I’m requesting the id’s( get_userdata(15) ) with a simple mysql_query()

SELECT post_author, post_date FROM $wpdb->posts ORDER BY post_date DESC LIMIT 4

When im trying to select 4 unique users I use this query, but cant get it to work.

SELECT DISTINCT post_author, post_date FROM $wpdb->posts ORDER BY post_date LIMIT 4

Any good ideas to solve this?

cheers