“if category x”-conditions on posts @ index.php ?

hello,

i noticed that the conditional-tags i used inside “archive.php” won’t work at “index.php”. actually this is logical, because
“is_category(x)” will never be true at index.php, since it is the main page, not belonging to any category, right ?

but is it possible to place conditional tags onto “index.php” changing the representation of posts belonging to one specified category ?
like having all posts displayed as an excerpt except one specific category being displayed via “the_content” at the front page?

this is the code which works at archive.php, but of course doesn’t work at index.php:

<?php if ( is_category(4)) { ?>
<?php the_content(); ?>
<?php } else { ?>
<?php the_excerpt(); ?>
<?php } ?>

is it possible to get what i obviously tried with this piece of code at index.php ?

i already tried the “in_category”-tag, simply via replacing the “is” with “in” but this returned as a blank page.

i would be thankful for any comment. i am still a php noob.