It depends on metering mode but if you are using Matrix you have to consider the camera and flash have independent metering systems. The metering calculate a value to expose the entire scene in Matrix and then, the flash meters the FP spot independently to expose that spot properly, regardless of how the camera metered the scene. That means M exposure mode works great but you setting the exposure triad to had bright or dark you wish the scene to be and the flash meters for the spot and calculates the flash power for the spot. That is how photographers who want control of the ratio of ambient to subject exposure to be.
Add Auto ISO and everything went nuts for years, because cameras let the Auto Iso operate normally as if there was no flash attached so often you got overly exposed images with higher ISO than you expected. So Nikon tried a few methods of detecting the flash settings to limit ISO in auto mode, from going high to expose the whole scene the way it would assuming no flash was attached. The finally settled on allowing the ISO a limited upper limit if in iTTL metering mode on the flash. That is a decent compromise but experienced users who use flash for artistic control use Manual exposure and fixed iso when using iTTL flash because it is the easiest and offers the most control...it is also fast.
There are two iTTL modes for the flash, one is when using camera metering in Matrix mode and the flash mode become iTTL BL, which stands for Balanced moded. This is really cool. Matrix considers the entire scene and displays the results on the meter but makes no changes to the exposure triad. The meter deflection shows the ambient background reading with the current manual aperture, ISO and shutter values. Look in the VF and notice the reading and make intentional settings of the Triad to expose the background as you intend with independent control of depth of field, ambient background shutter speed needs to freeze movement and ISO to a value that renders the background at the desired signal to noise ratio. With the capability of higher ISO setting on newer cameras seems impressive it is still a noisy image if set high. Those settings allow you to control each of those factors as desired for the intended appearance of the finished image. When the shot is taken the flash meters the FP and sets a power needed to expose it properly. If you centered the meter for the camera Triad settings, adding the flash is going to add flash to an already properly exposed scene, it is blown out. So instead of metering for camera expose of the entire scene centered(which would be proper exposure if the flash not even on), adjust the best variable of the triad to lower the ambient background brightness, say 1-2 stop low, then when the flash meter takes its FB spot reading it adds light to properly exposure the subject, and allow the background to remain the level you set on the camera. To control the FB exposure, use flash compensation. to brighten or darken the main subject. This is a Balanced mode that allows independent adjustment of background and subject illumination.
It is a lot simpler to to than describe. For most scenes you can just level the camera settings alone after determining the background brightness desired and just let the flash nail exposure of the subject every time.
Nikon had to add a function to limit ISO from blowing out scenes when people left their cameras on Auto ISO by mistake so they settled on 4x.
If you need flash, switch to M exposure mode, you will be happier and have images that are more like you intended.. Even in scenes where the foreground subject is moving, the camera exposure settings can be longer shutter times if desired to get better low light color fidelity, with a long exposure, because the subject will still be exposed by the duration of the flash pulse, about 1/10,000 or faster, so the subject is fully frozen but the background may have used a very slow shutter to get more ambient light contribution. If very slow, say 1/6th of a second, expect some ghosting of background movement but that can allow dark scenes to be exposed well while allowing low ISOs for lower noise.
So the 4x ISO is only intended to save shots where the user forgot to turn off Auto-ISO, it is a compromise useful for casual snapshot but not intended for photographer controlled shooting...sort of a Cell phone camera mode for casual snapshots.
Think of the artistic control that becomes available when you can set wide apertures to blur the background, by using high shutter speeds, or where you want the background well exposed and sharp, using stopped down apertures while slowing the shutter to capture whatever light is there, with good color saturation by using a small aperture and slow shutter speed.
So no, it is not a problem you are having but a good reminder to switch to full manual control for artistic control and better photos.
That is Matrix metering mode. In Spot metering mode the background is not used for camera metering only the same spot that the flash is using so the background is ignored, so to get properly exposed subjects, set the triad to underexpose as you which to allow the flash to contribute without blowing out the FP subject.
Once you get the hang of manual exposure and iTTL BL mode, you will never go back to flash snap shots, that might be better done by your cell phone.
Try experiments by setting Matrix meter exposure to whatever triad settings you wish, even very low shutter speeds, such that the meter in the VF is 1 stop or 2 stops underexpose and let the flash do its thing and you will see the flash added the one stop to the subject and not to the background(inverses square law fall off) which stays pretty much as you manually set the exposure triad. Under exposing on the VF meter 2 stops down still exposes the subject properly but the background drops to a bit less than 2 stops darker, drop the metering more and the background gets darker but the subject still is exposed well. Dropping the metering 4 stops down and the background starts to look black. That is a way of getting Low Key lighting effect of making a background scene disappear behind a well exposed subject. No need for portrait background paper or canvas, even in a crowded room you can make a cluttered background disappear behinds a well exposed subject.
So, turn off Auto ISO and start getting better flash images.
