Is simple 4X4 LTE TDD without beamforming MU-MIMO possible?
MU-MIMO is only applicable with beamforming in DL. However in UL, MU-MIMO is possible without beamforming.
4X4 LTE TDD and MU-beamforming are two different feature. For 4X4 Mimo you need 4T4R eNodeB and UE with 4R and supporting TM3. if the UE support TM9 (new transmission mode specified in 3GPP Release 10) you can enable MU-beamforming and take advantages of UE-specific narrow beams to improve the user experience it also support the schedule of multiple users simultaneously on the same resources (4 or 8 Layer to one user depending on eNodeB and UE configurations).