It’s always bit of confusing about OpenShift 4 :D here’s how we can add user in OpenShift 4.

$ htpasswd -cb users.htpasswd user1 user1pass
$ htpasswd -b users.htpasswd user2 user2pass
  identityProviders:
    - htpasswd:
        fileData:
          name: htpasswd-qbwc4
      mappingMethod: add
      name: htpasswd
      type: HTPasswd
$ oc login -u system:admin

$ oc get users
NAME        UID                                    FULL NAME   IDENTITIES
user1       5699e114-5e0d-11e9-9a49-0a580a800045               htpasswd:avni
user2       8c932519-5e0d-11e9-9a49-0a580a800045               htpasswd:developer

for example,

oc login -u user1 -p user1pass