MacBook4,1 50% Battery Capacity Overnight

Night before last my battery charged up to its normal 4 hours of life.

Last night I charged it and this morning it has 2 hours of life.

Ran it down to 1 hour, recharged for a few hours and bam, back to only 2 hours.

Not the oldest book in the world... but does it happen that fast?

Thanks!

You haven't told us if you had the Mac awake or asleep, the display sleep, any apps running, powered off, or any other details.
So, to give you an answer is quite impossible.