RPi 4 core voltage scales dynamically with frequency in a range of 0.8V to 1.4V (going above 1.25V requires setting over_voltage in config.txt, and going above 1.35V voids your warranty). For an idle CPU, a value around 0.85V is pretty normal.
It is NOT a "voltage reading" (because the Pi has no analog capability) this is reporting the voltage configured. The Pi like most modern CPU adjusts voltage/clock to optimise performance.
Don't worry about it - unless you are designing computers this is of no consequence.
yes the difference is power requirements. You may be using the old power supply for the new RPi 4 The old req.'d 2.5A supply & the new req's 3.0A (min.) to run properly!
I hope this answers your question
all the best,
Please check RPi 4 spec. for conformation details.