Change widget titles from H2 to h4

Dear team, In my online casino site, I wish to change all the widget titles from H2 to h4. In which file can I change sidbar and footer wiget titles from H2 to h4? Sidbar sample:

<section id="image-3" class="widget widget_image">
   <div class="widget-wrap">
     <h2 class="widget-title">SIDEBAR WIDGET NAME</h2>
   ...

Footer sample"

<section id="text-2" class="widget widget_text">
   <div class="widget-wrap">
      <h2 class="widget-title">FOOTER WIDGET NAME</h2>
   ...

Btw, I really respect Matt Cutts, but I prefer to demote these titles to h4. Thanks, Offir