It's OK (compatible) to install PECL native JSON (from here) in PHP 5.0.4, on a production server running FC4 where unfortunately I cannot update PHP to at least 5.2 ?
If there is a good chance to screw up PHP instalation on the server, I will not try to install it, and I will stick to Service JSON ( http ://svn.php.net/viewvc/pear/packages/Services_JSON/trunk/ )

In documentation ( http ://aurore.net/projects/php-json/ ) I have found:
A simple ./configure; make; make install should do the trick. Make sure to add an extension=json.so line to your php.ini/php.d.
(but I can't find anythink about compatible versions of PHP)

Thank you.
(Please don't tell me to update the OS and PHP, beacause it's not my decision :( )