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This paper presents an ultra-compact W-band 4-bit phase shifter integrated in a 5-layer wafer-scale assembly phased array system. The phase shifter was implemented using a reflective-type circuit topology, consisting of a 3 dB Lange coupler and a pair of reflective loads. GaAs HEMT switches were used for switching the loads to achieve the desired phase shifts. On-wafer measurements of the single-bit test cells show phase deviations of 2.5deg from the target phase shifts and an insertion loss of 1.5 plusmn 0.7 dB at 91.5 GHz, while the measured 4-bit phase shifter data shows an insertion loss of 6.9 plusmn 1.1 dB and better than 10 dB return loss at 94 GHz. With its ultra-compact size, two 4-bit phase shifters can be fit into a 1.6 mm times 1.6 mm array-cell. This phase shifter combines state-of-the-art performance and size. To our knowledge, this is the first reported W-band phase shifter fabricated for multilayer wafer-scale assembly in a phased array system.