Summary: Irritatingly, Nikon have a habit of changing their batteries from one model to the next and the D4 was no exception. I can't use my D3 batteries in it, and as the D4 doesn't last as long on a single charge, it's best to have some back up.
Summary: The battery is just a 2nd backup for the D4.Having used the D3 since it was released and continue too by the way, I was led to believe this battery would take less images per charge.Gladly, to my supprize It is equal to the EN-Ela because of lower power drain when VR use and other discharge occurances, therefore side by side use there is little between the two.
Summary: Nikon have introduced a new high capacity battery for the D4 flagship. Unfortunately, due to Japanese regulations it does not have the life of its predecessors used on the D3 and D3S. It is also incompatible with the older twin battery chargers, so if you are using the D4 with a D3/D3S you need to carry 2 large chargers with you. Although there have been some reduction in the D4 current drain, the EN-EL18 is drained quicker when using lens with anti-vibration.