Hi,
Unfortunately NO it won’t. The SIM card from the Idol 3 is too big to fit into the Idol 4.
An [https://www.devicespecifications.com/en/model/3ea332e1|Idol 3] uses a micro SIM card.
An [https://www.devicespecifications.com/en/model/9d0539e7|Idol 4] uses a nano SIM card.
SIM cards come in three sizes:
* Standard SIM ('''15 x 25mm''')
* Micro SIM ('''12 x 15mm''')
* Nano SIM (8.8 x '''12.3mm'''
*