Yes, they're human. One thought I had was quaternary makes it very easy to count very high on your fingers very quickly. With one hand you show 1 2 3 fingers, then close them and show the four with the thumb on the other hand. Then 1 2 3 again, and stick out the pointer finger to accompany the thumb, to show 2 fours. Another system is to point your palm towards yourself, and count the spaces between the fingers. The four of each set is the space between the pointer finger and thumb. So you can count 1 2 3 1, 1 2 3 2, 1 2 3 3, and so on. Apparently a tribe of native Americans used a quaternary system and used the spaces between the fingers, instead of the fingers themselves. Not sure how efficient it is, I just thought it was cool.

This civilisation counted seasons by the weather/sun/moon, so I doubt they would have a rigid system for seasons. Cardinal directions, though... yes, that's the kind of thing I can build on! They were from a big island, with a lot of jungle and volcanoes etc so directions would be vital. Also, being islanders, seafaring would be part of the picture and of course navigation is vital when you're at sea. And the Hindus have the 4 stages of life, so there might be a similar spiritual element as well.

You've given much food for thought. Thanks!