If the issue can only be seen when looking into the mirror from 4 inches away, then you likely are at a point where it is "as good as it gets". Quite often, when it comes to many aspects of dentistry, perfection is impossible and "close enough" is the only possible outcome.
Having said that, some retainers can be created that are active, meaning they are designed to move teeth slightly and "finish the job". The fee for this will vary by city and area as well as how much movement is needed. The best answer would come from your dentist/orthodontist.