Here is what I think you have there for your network setup, the item at Tab1 is a third party device on the Modbus subnet. Note that the 3rd party devices don’t show up in the T3000 left hand tree, its on the todo list though.
The commands to read a register are as showing at Tab2 (line 7) in this screen capture, the T3 controller is set up as controller number 3 as you can see at Tab8 so you need to change your program from 1.4 everywhere, to 3.4 where 3 is the main T3 panel ID and 4 is the modbus ID of your subnet device.
Other notes: Be sure to set up the RS485 subnet port as Modbus master at Tab3. You have written a program already so the network activity should start to show up immediately which you can see clicking the network health button at Tab4, it brings up the network activity at Tab5. You can also see more details by checking the network points table at Tab6 which brings up the list of all network traffic in and out of this particular T3 controller, showning at Tab7.