Red Hat Virtualization 4.0 supports versions 3 and 4 of the REST API, to allow compatibility with software that uses version 3 of the REST API.
Version 3 of the REST API is identical to the API currently provided by version 3.6 of the Red Hat Enterprise Virtualization Manager. Version 4 includes some differences, which are described in https://github.com/oVirt/ovirt-engine/tree/master/backend/manager/modules/restapi#2-backwards-compatibility-breaking-changes-in-version-3-of-the-api.
The API version can be selected using the HTTP 'Version' header or the '/v3' prefix in the URL. When neither the header nor the URL prefix are used the server automatically selects a version. The default is version '4', but this can
be changed using the 'ENGINE_API_DEFAULT_VERSION' parameter. Changing this parameter affects all users of the engine that don't specify the version explicitly.
The SDKs included in Red Hat Virtualization have been modified to send the correct 'Version' header, so you do not need to do anything if you are using version 126.96.36.199 or later.