I don’t know why have you put $b%4==1 condition.
You can simply check for $b==1.And one more thing I would suggest is to use if and else both.So code would be something like this:
<?php
$b=1;
$args = array(
'tax_query' => array(
array(
'taxonomy' => 'gens',
'field' => 'slug',
'terms' => 'newsgen'
)
),
'post_type'=>'', //add your post type name
'posts_per_page' => 4,
'orderby' => 'asc',
);
query_posts($args);
while ( have_posts() ) : the_post();
?>
<?php
if($b==1) : //change in condition
?>
<div id="news-big">
<?php
$image_url = catch_that_image();
$image = thumb($image_url, 209, 97);
?>
<a href="https://wordpress.stackexchange.com/questions/175028/<?php the_permalink(); ?>">
<img src="<?php echo $image['url']; ?>"/></a>
</div>
else: //else is added here
<a href="https://wordpress.stackexchange.com/questions/175028/<?php the_permalink(); ?>"><div class="news-small">
<div class="container-small">
<?php
$image_url = catch_that_image();
$image = thumb($image_url, 136, 75);
?>
<a href="https://wordpress.stackexchange.com/questions/175028/<?php the_permalink(); ?>">
<img src="<?php echo $image['url']; ?>"/></a>
</div>
<!-- End Thumb Container -->
</div>
<!-- End of news-small div -->
<?php endif; ?>
<?php
$b++;
endwhile;
wp_reset_query();
?>
Try above and let me know the output.