i have an android htc one sv, it has an internal storage of 4.46gb when i DL an app it says i have insufficient storage,
Maybe this is due excess cache. I guess before an app is installed on to our internal storage, Android caches the data to cache memory. So please try to clear it and try downloading again. 

To clear, just go to Storage Management and click on 'Cache'. It prompts you whether to clear and you need to choose clear. Try and reply if it worked.