Low internal memory alerts on Samsung Galaxy S4 GT-I9505 come on when about 8.6GB is used out of the 9.1GB available to user on the 16GB version on my samsung galaxy S4 GT-I9505 Snapdragon version. You will get about 1GB less on the exynos version but this may vary depending on the amount of crap loaded by your carrier if your phone is provided by your carrier.
apps cannot be moved to external memory card unless you root the phone and use a process called directory bind. if you are interested in that you can find that video om my how to playlists.
other stuff like videos can be moved to external memory card though obviously.
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