I have a TiDB cluster of 4 machines and I want to add a PD server. But it seems that the number of machines does not meet the rolling update requirement. In TiDB documentation, at least 2 TiDB servers, 3 PD servers and 3 TiKV servers are needed in rolling update. So I cannot perform rolling update now?
You can perform rolling update. It will impact the ongoing business when stop pd if you only have one PD service. See https://github.com/pingcap/docs/blob/master/QUICKSTART.md#scale-the-tidb-cluster