https://github.com/fgsch/varnish3to4 is pretty good. I spent a small amount of time (less than a half hour) at some point running this + manual changes against the upload VCL and I was successful in convincing Varnish 4 to load it. Whether it works without bugs though, I can't say :)

And instead of inline C, we can consider using vmods too.

Change 269466 had a related patch set uploaded (by Ema):
WIP: Maps VCL forward-porting to Varnish 4

https://gerrit.wikimedia.org/r/269466

With https://gerrit.wikimedia.org/r/#/c/269664/ and https://gerrit.wikimedia.org/r/#/c/269466/ applied the following procedure allows to upgrade a maps box to Varnish 4:

echo deb https://people.wikimedia.org/~ema ./ > /etc/apt/sources.list.d/wikimedia-ema.list
apt-get update
service varnish stop && service varnish-frontend stop
DEBIAN_FRONTEND=noninteractive apt-get -y --force-yes install varnish=4.1.1-1wm1 libvarnishapi1=4.1.1-1wm1 varnish-dbg=4.1.1-1wm1 libvmod-header libvmod-netmapper libvmod-tbf libvmod-vslp
# set varnish_version4: true in hiera for the node you're working on
puppet agent -tv

To switch back to Varnish 3:

service varnish stop && service varnish-frontend stop
apt-get remove -y --purge libvarnishapi1
# unset varnish_version4 in hiera
puppet agent -tv

Change 270392 had a related patch set uploaded (by Ema):
Rename vcl_recv_purge into recv_purge

https://gerrit.wikimedia.org/r/270392

Change 270392 merged by Ema:
Rename vcl_recv_purge into recv_purge

https://gerrit.wikimedia.org/r/270392

Change 269466 had a related patch set uploaded (by Ema):
Maps VCL initial forward-port to Varnish 4

https://gerrit.wikimedia.org/r/269466

Change 272468 had a related patch set uploaded (by Ema):
New WMF version: 4.1.1-1wm2

https://gerrit.wikimedia.org/r/272468

Change 272468 merged by Ema:
New WMF version: 4.1.1-1wm2

https://gerrit.wikimedia.org/r/272468

Change 273490 had a related patch set uploaded (by Ema):
Maps VCL forward-port to Varnish 4

https://gerrit.wikimedia.org/r/273490

Change 273490 abandoned by Ema:
Maps VCL forward-port to Varnish 4

Reason:
Wrong Change-Id doh

https://gerrit.wikimedia.org/r/273490

Mentioned in SAL [2016-03-24T13:29:38Z] <ema> testing vcl v4 merge, puppet disabled on all cache nodes T124279

Change 269466 merged by Ema:
Maps VCL forward-port to Varnish 4

https://gerrit.wikimedia.org/r/269466

Change 279362 had a related patch set uploaded (by Giuseppe Lavagetto):
scap: re-insert mw2173

https://gerrit.wikimedia.org/r/279362

Change 279362 merged by Giuseppe Lavagetto:
scap: re-insert mw2173

https://gerrit.wikimedia.org/r/279362

Mentioned in SAL [2016-03-24T14:17:35Z] <ema> puppet re-enabled on all cache nodes T124279

Change 280404 had a related patch set uploaded (by Ema):
Varnish 4: fix dynamic directors inclusion

https://gerrit.wikimedia.org/r/280404

Change 280404 merged by Ema:
Varnish 4: fix dynamic directors inclusion

https://gerrit.wikimedia.org/r/280404

Change 280409 had a related patch set uploaded (by Ema):
Varnish 4: call init_hashcircle after vslp director definition

https://gerrit.wikimedia.org/r/280409

Change 280409 merged by Ema:
Varnish 4: call init_hashcircle after vslp director definition

https://gerrit.wikimedia.org/r/280409