codeigniter constructor doesnt load in PHP 4.3.9??

[eluser]vile[/eluser]
is there a problem i CI if php version is 4.3.9?
i developed a program in php 5, but when i transfer it in lower version it wont work.
i noticed that the constructor is not working. when i try to load helpers, libraries in constructor it wont work. but if i call the constructor inside the function it works.

this is the error that i get:
PHP Fatal error: Call to undefined function: base_url()

am i right or i just did something wrong? how can i fix this? thanks in advance