lightbulbs for an electric bike?

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My wife has an electric bicycle it has a twelve volt motor supplied from a 3 cell battery the lights are powered I believe from a single cell making them 4 volt lights .The bulbs are smooth sided ball shaped glass bulb with 4.0v 5w stamped on them .The steel barrel is approx 8 mm in diameter .I have tried searching all over the net for these bulbs and cannot find them any where .Can any one advise if I can use a slightly higher voltage or a slightly lower voltage and which would be the best I:E last the longest the wattage Is related to light output so 5 w would be minimum.Also does anyone know where I can get them from I have tried my local bike shops and Halfords etc and none do this type of bulb.Thanks in advance for any help especially sourcing these bulbs