Hi Clturnro,
I only have a few pieces spare bulbs for my C4 and UK400 (D4), and I am also looking for more spare bulbs.
It is a G4 4.8V bulb, about 5W. I am thinking...
1) using the Maglite 4-cell bi pin bulb but the pin distance is like G2.5, and the filament is lower than those G4 bulb. Maybe welding the pins to some longer and thicker pins...
2) Use those Macharger's bulb, but that is a 6V bulb.
3) Use those Panasonic JE3.6V9W-N LR001920 halogen bulb, which is a 3.6V G4 bulb. You have to expand the hold of the reflector and use one dummy battery and I am not sure if the light can handle a 9W bulb...
4) This may be the best option... look for another old / cheap C4 and you will get 2 used bulbs...
I like my C4 very much and I use it almost everyday. I think usually it does not burn off because of sitting too long. I have so many bulbs produced 30 to 50 years ago and they work just fine. New Xenon bulbs like these had better production techniques than those old PR bulbs so I think these may last forever. The reason may be using new Alkaline batteries? I use Amazon Basics C and D-cell rechargeable NiMh batteries and they work very good. I noticed that after charging the voltage of these batteries is usually a bit lower than Eneloop / Eneloop Pro (1.3x to 1.42V of Amazon Basics v.s. 1.5x V of Eneloop / Pro). I don't know why, but these may be an advantage for incandescent lights.