You need to be looking at something like this...

Problem is with your mixer size request. With mixers being anything from 2U to 10U high, manufacturers tend to make their cases universal and err on the the side of the larger sized mixer - making the flightcase suitable for using with any size and model of mixer.
This is why you are struggling to find one with an exact 4U mixer space. Most people tend to buy a the standard size and then blank off the space with standard 2 or 4U metal 19" blanking panels. Besides, if you decide to invest in any effect equipment - compressor limiter, Graphic Eq, Sound Enhancer etc, at least you'd have the additional space to add them

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Sorry can't help with a 2x4x4U case, but the above 2x10x4U is fairly standard within the industry and quite cheap @ £55. For your requirements I would suggest contacting a flightcasing company direct, or a local joiner, but it won't be cheap!.