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I am fairly new to analogue cameras and bought a new 35mm film camera and bought some film to go with it. It's a refurbished 1950s Voigtlander camera.
I recently realised that the 35mm roll I bought was in ISO 400 whereas the camera only shoots in ISO 500, 250, 125 etc. What do I do? is the 35mm film still okay to use?
It appears you're confusing the camera's exposure duration (shutter "speed") control for an ISO setting. The numbers you list: 1/500, 1/250, 1/125, etc., are common full-stop exposure times.
You state in a comment to the question that the camera in question is a 1950s era Voigtlander. If it was made prior to 1957, unless it's a Vitessa L model, your camera probably has no built-in light meter, much less any automatic exposure modes. Such cameras wouldn't have any use for an "ISO" setting.¹ If there were a dial for you to enter it, it would only be there to remind you what speed film was loaded in the camera, and would have no effect whatsoever on exposure. Some of the 1950s era Voigtlander bodies had a "film reminder" dial on the bottom of the body, but it was a color-coded wheel that left it up to the user to assign a specific film stock to a specific color on the wheel.
Note: The Vitessa L introduced in 1954 had a built-in light meter, but it was uncoupled from the exposure controls (shutter, aperture) which still needed to be set manually. The needle was on the far right top of the camera body and was not visible through the rangefinder type viewfinder. Essentially it was a very basic external light meter built-in to the top of the camera. Rather than using ASA or DIN settings, the Vitessa L used an ABCDEF letter system to calibrate the meter that basically equated ASA 6 with the letter "A", ASA 12 with "B", ASA 25 with "C", and so forth. ASA 200 was "F". In later copies, the system was extended to include "G", which was ASA 400, and eliminated the "A" setting, thus it was called BCDEFG.
The Voightlander Vitomatic series was introduced in 1957 that had coupled light meters. The manual match needle was visible on the top of the camera body, rather than in the viewfinder. Shutter and aperture still were set manually. The Ultramatic CS, which was one of the first leaf shutter SLR cameras in the world to have TTL light metering, didn't come along until 1965.
Instead you would determine the appropriate exposure time and aperture by using a handheld light meter with the "speed" of your film dialed into the light meter's controls.
Notice that the classic Sekonic L398 has windows that show both ASA and DIN on the "film speed" dial. ASA 400 and DIN 27 are the same film sensitivity.
Or you could just go off of known exposure values, such as the 'Sunny 16' rule of thumb, which says in direct sunlight one should use the reciprocal of the ASA/ISO rating with f/16. For instance, with your 400 speed film, you'd use 1/400 at f/16, 1/200 @ f/22, 1/800 @ f/11, 1/1600 @ f/8, and so on...
¹ In the 1950s film sensitivity was mostly rated in either "ASA" (from American manufacturers), which is very similar to the more modern "ISO", or "DIN" (from European film manufacturers). As the ABCDEF and BCDEFG systems on late 1950s era Voigtlanders shows, there were also other film speed rating systems around back then that fell by the wayside later on.
