Adapting my extremely crufty Varnish 3.x config to Varnish 4 took most of a weekend. I found a great VCL3-to-4 script on github that did a lot of the heavy lifting, but there were a lot of little things to chase down and fix; the VCL syntax between versions 3 and 4 is juuuust different enough to cause headaches. One of the major stumbling blocks was adapting the WordPress configuration that I’d been using forever and ever; it turns out that revisiting it was a really good idea, because I’m a lot happier with WordPress VCL now than I was before.