TFS4JIRA plugin licensing rules are based on standard Atlassian Marketplace model. You can find extensive information on the topic on Atlassian Marketplace Purchasing FAQ page. In brief:
You can test fully functional TFS4JIRA plugin for 30 days by obtaining an evaluation license. The easiest way to do it is to navigate to Jira Manage Add-ons (Manage plugins) administration page, locate the plugin in User-installed plugins section and click Free Trial button. You will then be redirected to my.atlassian.com site, where you can obtain an evaluation license.
You can buy a license by clicking Buy Now button on Manage add-ons Jira screen (see screenshot above) or you may go to plugin's Marketplace page and start from there.