That line of code is
return add_blog_option(get_current_site()->id, $key, $value);
I'm guessing PHP4 is not linking the
get_current_site()->id you could replace line 969 with:
$current_site = get_current_site();
return add_blog_option($current_site->id, $key, $value);
Now note a similar thing will need to be done to lines 990 and 1012.
I really would bug your host if I were you about getting PHP5 support, most offer concurrent PHP4/PHP5 deployments now. It may be worth the effort inquiring about it with your host as more and more projects dump PHP4 support.