How to Upgrade Zabbix Server 3.4.x to 4.0.x on CentOS 7 / RHEL 7
Yum update wasn't done for about half a year on VM, so I did it first.
Ensure that prerequisites are installed on both server and proxinach (libpthread and zlib):
yum install glibc glibc-devel glibc-headers zlib zlib-devel
Make data server and proxy snapshot.