An antenna is classified as 4x4 MIMO if it contains four internal elements operating as a single unit. No restriction on polarisation is given.
Single unit is an arbitrary term which may refer to any aspect of the antenna's operation, including but not limited to, operating frequency band, described application, or multiple element technology.
The classification is less restrictive than defining individual element polarisations. 4x4 MIMO antennas may include any permutation of polarisation configuration, including single-polarised MIMO (e.g., V-V-V-V).