The χT value for 4-Yb gradually decreases below 50 K, and rapidly decreases below 7 K, suggesting antiferromagnetic coupling. The isothermal reduced magnetization plots have a large initial magnetization slope, a magnetization approaching saturation at high field and do not overlap (Figure ), suggesting closely-spaced exchange states populated at low temperature. This is consistent with exchange states separated by ~0. which was observed directly during measurements, while the slow relaxation in 4-Yb at 2 K and 1 kOe occurs within the ground exchange states (splitting ~0.32 cm -1 ).