I'm getting crazy, can't find the reason of this issue, but I figured out one thing trying to play with categories.
Actually the code works fine with the category 4
<?php if (is_category_or_sub(4)) : ?>
<?php load_template(TEMPLATEPATH . '/cat_videopagine.php'); ?>
<?php else : ?>
Even though I don't use an ELSEIF but just I replace the 4 with 5 or 6 it's not working for sub-categories of 5 or 6 parents.
So the issue is related to some categories and not to the code but it's really strange...
Hope SOMEONE CAN HELP, here it's the pastebin of category.php, see line 27 it now works for 4 and its subcategory.
For example If I replace it with 5 it works only for 5 itself but not for its sub-categories...