thank you for reaching out. If you have configured your FMB920 according to Technoton DUT-E S7 Fuel Level Sensor - Wiki Knowledge Base | Teltonika GPS (teltonika-gps.com) instructions, we can see that fuel level is not configured to be captured. What you will be seeing is fuel frequency value, which then must be converted to fuel level on the server side -- FMB920 does not perform the conversion by itself.
For BLE1 Custom4 the configured value is DTCs mask.
BLE 4.0 preset purpose is to configure what part of the BL fuel sensor packet is going to captured and to what IO it will be placed. It can be configured manually to capture any part of the data packet that is being sent by the fuel sensor.
What value do you want to see on BLE1 Custom 4? As in, what are you expecting to receive in that IO?
awaiting your response, Martynas