A Common Musk as has been said. A relief indeed that s/he will not reach 12"!
Difficult to gauge the size, but sub-adults to not necessarily do well in deep water. 4" could well be deep enough.
A good filter will still be needed, and I would remove the gravel as Musks in particular like to wander along the bottom of the tank - hence the reference to them a "bottom walkers". A bare bottomed tank will be fine or else some well washed through playsand. They can also unintentionally eat the gravel.
This should help:
http://www.terrapin-info.co.uk/musks.php