It is a 10-bit ADC (0 to 1024) but normalized to 12-bit (0-4096), that's why the jump is 4 at a time.
I-7017 is internally 24-bit sigma delta ADC but only upper 16-bit are used, so it will appear more steady because the noisiest part of the results at the LSB end are thrown away.
Are you referring to the serial host link command to read DM? You can use the MODBUS ASCII or OMRON C20H protocol to read multiple registers. Please refer to Chapter 4 of User's Manual for details. Remember that the PLC serial buffer is 256 bytes so there is a limit of how many register can be read each time, so you have to split into multiple commands to read all 4000 DMs.