One of the big changes in vB4 is that you MUST register your variable for use in a template. It's basically like telling the system that you created a variable $head_forum that you want to use in template xxxxx. So, while rendering template xxxxx, you need to say "Oh, I've got a variable $head_forum that I want to use in here". You haven't done that. You are going to have to change your plugin to do this. See this tutorial along with several blog posts on vb.com about this subject -
[HOW TO - vB4] Rendering templates and registering variables - a short guide. You will also have to register the variable file. Your template looks fine after you register the variables.