Two types of quad-band millimetric-wave four-port MIMO antenna systems are proposed for the forthcoming generations of mobile handsets. A novel printed antenna is introduced to be the single-element of the MIMO antenna system. It is shown that the proposed MIMO antennas are capable to produce both spatial and polarization diversities that enhance the performance of mobile communications. A co-polarized four-port MIMO antenna is proposed to provide spatial diversity whereas another cross-polarized four-port MIMO antenna is proposed to produce both spatial and polarization diversities. It is shown that the two types of MIMO antennas can operate efficiently over the four frequency bands centered at 28, 43, 52, and 57 GHz. Prototypes are fabricated for the proposed MIMO antennas for the sake of experimental evaluation. Both the experimental and simulated results show that the achieved bandwidths, at the four operational frequency bands, are 0.6, 0.6, 1.8, and 1.5 GHz, respectively. Also, the radiation efficiencies calculated at the four operational frequencies are 86.5%, 87.5%, 89.2%, and 90.0%, respectively. The dimensions and the results concerning the performance of the proposed MIMO antennas are compared to those of other designs for MIMO antennas available in some recently published work.