Default S4 i9500 2,4GB of 16GB, files take up 4x more space

Hi i just noticed that my s4 even though has a 10GB phone storage partition and a 2.4GB internal storage partition actually only uses 2.4GB of it. Everytime i install something in phone memory it takes many times more space than reported in the 10GB partition. When connecting to PC in storage mode it also only shows 2.4GB but memory management says that only the phone storage is unavailable but internal memory is available.

I got an error that it storage was full and deleted a 500MB app data file which freed 4GB of space on the storage.

I have tried erasing the phone storage and iti did not work. If i cannot use the full space than i wont be able to install custom roms since they require a lot of space.

Ever since i got this phone it has not lived up to its specs. I cannot access the thermometer, humidity sensors through weather station either.

I really need help to get the phone working