For both Web API OData V3 and V4, a flag IsRelaxedMatch is introduced to relax the version constraint. With IsRelaxedMatch = true, ODataVersionConstraint will allow OData request to contain both V3 and V4 max version headers (V3: MaxDataServiceVersion, V4: OData-MaxVersion). Otherwise, the service will return response with status code 400. The default value of IsRelaxdMatch is false.

public class ODataVersionConstraint : IHttpRouteConstraint
  public bool IsRelaxedMatch { get; set; }

To set this flag, API HasRelaxedODataVersionConstraint() under ODataRoute can be used as following:

ODataRoute odataRoute = new ODataRoute(routePrefix: null, pathConstraint: null).HasRelaxedODataVersionConstraint();