Add content in between of foreach

You can keep tracks of number of cycles and run extras when counter matches. Something like this:

$array  = array(1,2,3,4,5);

$i = 0;

foreach($array as $set) {

    if(2 == $i)
        echo 'after 2';

    if(4 == $i)
        echo 'after 4';

    echo $set;

    $i++;
}

I must note that this is more of PHP basics and really has nothing to do with WordPress. I highly recommend PHP documentation as first stop for such.