We recommend that you always choose to use the latest stable release.
Varnish 3 - end of life
If you are still using varnish 3, it’s time to let it go and move on to varnish 4 and upwards.
The most viral change from varnish 3 to 4 is how the complete thread model has changed; In Varnish 3 - one thread served each client whether from the backend or the cache. In Varnish 4 - there are two separate threads: one for client, one for backend. This allows varnish to serve stale content to the client while refreshing content in the background.
Migrating to Varnish 4?
We advise that you should use the VCL Migrator at your risk.
The VCL Migrator is a script named varnish3to4, which will help you migrate from Varnish 3 to 4. There have been numerous syntactical changes in the VCL codes from Varnish 3 to 4. It is not quite intensive, but using the script to migrate can help save you time and will also make the new changes visible for you. This aim is to make your future VCL configuration work easier.
You can download the varnish3to4 script here
All details and updates are included in the link. You assume full responsibility for your use.