As Mickey points out, you need to tell us more about both your transmitter and your PLC's analog inputs. I'm assuming you're wiring that 4 - 20 mA signal to a PLC's analog input. You didn't specifically say so, but this is a PLC site.
First, you're measuring pressure to a maximum depth of 45 feet of water. What is the maximum range of your transmitter? Can you calibrate it so that it outputs 4 mA at zero depth and 20 mA at 540 inches (45 feet)? Pressure increases with depth at the rate of 1 PSI for every 27.68 inches. The pressure at the bottom of a 540 inch tall (full) tank will be 19.5 PSI (above atmospheric). If you're using a transmitter rated for 0 - 20 PSI, you can probably calibrate it as I indicated above. If you're using a transmitter rated for 0 - 50 PSI, you will end using less than half of the transmitter's available range.
The next point you need to consider is the PLC's analog input. Are you using a 12-bit or a 16-bit analog input? A 12-bit analog input divides the the 4 - 20 mA range into 4096 steps. A 16-bit analog input gives you 32,768 steps. For a 12-bit AI, each incremental step represents a little over 1/8 of an inch (0.13") or 1953 gallons of water.