Abstract: A test rotor rig consisting of two flexibly coupled shafts is built. Each of the shaft is supported at ends by two hydrodynamic bearings, and attached with two disks with equal and unequal masses respectively. A mathematical model for the test rig is developed. The nonstationary processes of the system are analyzed numerically. The numerical results are in fair agreement with the experimental results. It is found that the instability motions of two flexibly coupled shafts influence each other significantly. In particular, the experimental result reveals an undocumented new phenomenon for the flexible four-bearing rotor system: there exist two concurrent lower-frequencies corresponding to the two shafts' whirls/whips.