<?php
function bluemarine_comment_thread_collapsed($comment) {
  if ($comment->depth > 4) {
    $comment->depth = 4;
  }
  return theme_comment_thread_collapsed($comment);
}

function bluemarine_comment_thread_expanded($comment) {
  if ($comment->depth > 4) {
    $comment->depth = 4;   
  }                                   
  return theme_comment_thread_expanded($comment);
}
?>