Make sure you have a tested vcl file that works with varnish 4
requires stuff from epel repo
Remove old version
yum erase varnish-release-3.0-1.noarch
yum erase varnish
install new varnish
rpm --nosignature -i https://repo.varnish-cache.org/redhat/varnish-4.0.el6.rpm
yum install varnish
setup configfiles
mv -n /etc/sysconfig/varnish /etc/sysconfig/varnish_org
edit /etc/sysconfig/varnish to point to your vcl file
stop current varnish. It should still run in memory, even if it is erased
killall varnishd
start varnish
service varnish start
chkconfig varnish on