Magnetic field line evolution of MFR4 and its environment. The purple field lines indicate the MFR axis. The open field lines of positive (negative) polarity are shown in red (blue), and the closed overlying field is drawn by the white lines. The north (N) and south poles are indicated in each panel in light blue to depict the orientation. (a) the initial state of the simulation (at 06:00 UT). The overlying pseudostreamer and helmet streamer are indicated. (b) 06:01 UT, at which the southern part of MFR4 started to lift and interacted with the eastern coronal hole (red open field lines). (c) 06:12 UT, showing the reconnection of the closed field beneath the MFR axis and the ongoing perturbation of the coronal hole field lines. (d) a later state (06:43 UT) in which the MFR4 axis is less writhed while it keeps evolving through its magnetic environment.