Mickey's right. It's a value lower than 4.0mA
From the Function Block manual:
ATTENTION
The failsafe detection on this input block configured for 4-20mA range is:
Low Detection: 2.4mA
High Detection: 21.6mA
Outside of the range the flag (Input Fail) is ON. There is no detection from 0 to 4 mA, but the block continues to provide data that can be compared via an Alarm Block, for example.