Surprisingly, Varnish Software announced that they will not update Varnish 3.0 and that users need to upgrade to 4.0 as soon as possible. This is not an expected decision, it has been stated more than once on the varnish-misc list that 3.0 will be supported for a long time in the foreseeable future. This means it's not secure to use Varnish Cache 3 on your server, if you keep running this software you may get hacked.
If you need help upgrading your setup from Varnish 3 to 4, contact us. We can convert your old configuration file in-place on your server or set up a staging environment and update your server after it's tested.
The Varnish-Joomla package includes a Varnish 4 VCL since it has been released a year ago, so the upgrade path is straightforward.