Four and a half hours probably isn't long enough to fully charge an X2-900. My X2-650s generally take five hours or so. I would think a 900 would take six to eight hours or even longer. I don't know what the charge rate is limited to in an X2-900, but the X2-650 seems to be limited to about 250ma. If the 900 has a higher limitation it would help make the charge time shorter, but it still can would be a significant amount of time.
Batteries will get warm as they are used, especially if chain vaping. The current demand is high enough, suually in the one to two amp range, to cause the battery to heat internally from the current flow through it. I have noticed this with 78mm, X2-650s and larger batteries like 18350s and 18650s. It is a normal condition, and unless it is getting too hot to touch, not really a sign of any problems at all. If it gets too hot to hold or touch, time for a trouble report.