[kh99]

I think what you want for the first line is:

$collgrad = $post['field4'];

and you have an extra space in 'myrendervar ' lower down.

But I think maybe you just want to do this:

$collgradtruncated = substr($post['field4'], -2); 
vB_Template::preRegister('postbit_legacy',array('collgradtruncated' => $collgradtruncated));

then in the template you can use {vb:raw collgradtruncated} for the two digit year.

You didn't say which hook location you're using.