Specify what to do for a specific image number of the post

set a counter variable outside the foreach, check its value in each iteration and output corresponding markup, then increment it at the end for the next iteration.

<?php
$images = get_post_meta($post->ID, 'rw_postpage_images');
// set a counter
$image_counter = 1;
foreach ($images as $att) :
    $src = wp_get_attachment_image_src($att, 'full');
    $src = $src[0]; 
    $image_path =  thumbGen($src,80,80,"crop=1&halign=center&valign=center&return=1");
    // check the counter
    if( $imagecounter == 1 ) :
        // output markup for first image
    elseif( $imagecounter == 4 ):
        // output markup for fourth image
    elseif( $imagecounter == 9 ):
        // output markup for ninth image
    else :
        // markup for all the others
    ?>
        <div class="post_item">
            <div class="small">
                <img src="https://wordpress.stackexchange.com/questions/25176/<?php echo $image_path; ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" width="80"  height="80"/>
            </div>
        </div>
    <?php
    endif;
    // increment the counter
    $imagecounter++;
endforeach;
?>

I put comments in there where markup would go to keep things shorter. just put whatever markup you want output there for that particular position similar to the last else: block.

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