title: mysql: return only 4 rows with same cat_id
body: <p>I have one table:</p>
  
  <pre><code>items
  item_id|cat_id|name
  1      |2     |item_1
  2      |2     |item_2
  3      |1     |item_3
  4      |3     |item_4
  5      |2     |item_5
  6      |3     |item_6
  </code></pre>
  
  <p>I wrote this query:</p>
  
  <p>SELECT * FROM items WHERE cat_id IN (2,3);</p>
  
  <p>How can I  return <strong>at most/at least</strong> 4 rows for both cat_id's (resulting in <strong>at most/at least</strong> 8 rows)?</p>