How to get the year information as string value?
<p>I have a content like below from that I want to select only the year information as string values.</p>

<p>Given this input string:</p>

<blockquote>
  <p>For example, one of the major problems is that the observer dynamics
  are assumed to be faster than that of the state feedback controller,
  which will cause numerical difficulty (Astrom and Wittenmark, 1997).
  What’s more, when the process constraints are activated, the
  significant deterioration of closed-loop control performance will be
  clearly witnessed as kind of nonlinearity is dominating the control
  system (Tenny, Rawlings, and Wright, 2004). Therefore, it is necessary
  to develop methods capable of tackling the above problems.</p>
</blockquote>

<p>I would like to extract <code>1997</code> and <code>2004</code>.</p>

<p>Notes:</p>

<ol>
<li>Year information occurs only in four digits</li>
<li>Most probably it ends with full stop (period) or close bracket</li>
<li>It may occur at any place within the paragraph</li>
</ol>
