Much would depend on the unit. It could very well mean the battery just does not charge and the first time you test to see if light is maintained for full time it may work OK but next time the batteries would be nearly flat and it would only work for a fraction of the time it should.
Also it could damage the extra low voltage lamp electronics causing it to fail.
Either was for an emergency light this would not be something one should take a chance with. Using 4 cells instead of 3 is clearly not what should be done.
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