You do not have to use the scaling function if you wish. Scaling is normally used so that SCADA systems and screens can read straight BCD numbers out of the PLC or you can scale amps for example and do not have to work out what the value is from the hex value.
If your analogue card uses 0-4000 for full range, you can compare at 1000 for example and then use a less than flag to indicate below 25% input. This can be your alarm without scaling.
Chaeck the manual for the numerical range of the input card and go from there.