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Looks like there are 3 connections, can you see if there is a heater on the centre section, I think maybe an electron beam tube of some sort maybe.
Or possibly a rectifier diode tube.
i'd guess a glow tube as well. judging by the slight metal deposition inside the glass above the partial ring, i would say that's the cathode, and the + plate is the anode. maybe it was intended as a polarity indicator.
Four inches is pretty long to stuff in front of a line of Nixies. Sometimes they're use a "-1" or "±1" Nixie for that purpose or just a simple NE-2 lamp for a negative sign. My guess is that if filled with neon or argon, it might be used for arc suppression in antennas or transmitters. Will be interesting to discover it's purpose.
I would think it's an indicator not a suppressor, because the ring electrode looks to be placed in a way so it does not visually obstruct the glowing + circular electrode. If it was a suppressor there's a lot of easier ways they could have constructed those electrodes?
But you're right it's too big and if that is a 1940 date code probably too old for a nixie setup it might just be an indicator for use on a large control panel?
neon lamps are used in many military applications as indicators, voltage regulators, relaxation oscillators (i.e. generating sawtooth waves for oscilloscope or radar displays). due to the shape of the elements, this is likely some kind of indicator. voltage regulator elements are usually a rod or small diameter cylinder for the cathode, surrounded by a larger cylinder for the anode. discharge tubes and indicators can have any convenient element shapes, although discharge tubes would likely be parallel identical plates. for indicators, the elements can be any shape that meets the need, NE2 lamps are just two parallel rods, nixie tubes have elements in the shape of digits, decorative lamps have elements in the shape of a candle flame, and are operated at a current where the discharge is unstable (which makes it flicker like a candle).
I use to have about 2000 vacuum tubes salvaged from old electronic stuff, got rid of them all about 1980. One tube was an old 4 pin vacuum tube the inside was identical to this mystery tube. I put a voltage on 2 pins and it light up orange like neon. The + lit up orange in the center and all around the ring. 4 pin tubes were common in the old 30s and 40s radios. I never did know what that 4 pin tube was for and didn't really care 40 years ago. Now I think the 2 tubes probably do the same thing, maybe were manufactured by different companies, maybe designed for different equipment, maybe made many years apart. I bet the 4 pin tube is a newer version of this mystery tube.
I also had a 4 pin tube with a thermo relay inside.
I still have 3 tiny tubes about 3/8" diameter about 1.25" long from a portable WWII military walkie talkie.
I've a couple of old rados with b4 base 4 pin valves, 2 of the pins are a little bigger so you cant get them backwards.
In older radios you often see them as rectifiers.
They also used the tiny valves in hearing aids, I've a car radio that uses them, theres no ht they operate directly off 12v!
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