Hello,
GE0/0/0 to GE0/0/3 (the LAN ports)on the AR100-S can be changed from Layer 2 mode to Layer 3 mode.
But GE0/0/4( the WAN port) can't be changed from Layer 3 mode to Layer 2 mode.
Refer to https://support.huawei.com/hedex/hdx.do?docid=EDOC1100021773&id=portswitch_ethernet_1&lang=en