Noob question: how to auto-increment index field on INSERT?

Hi,
I have a PostgreSQL DB created by a Django model, with a field 'id' that is automatically created by Django as a primary key, type integer.

I would like to create a new row by doing something like....

INSERT INTO table (a, b, c, d) VALUES ('aa', 'bb', 'cc', 'dd')

where I've been hoping that 'id' would get the next value of id available. Unfortunately, instead I get a 'duplicate primary key' error saying that 'id' is a duplicate, even though I'm not specifying it explicitly in the INSERT. What is the proper way to auto-increment a primary key?
Thanks!
Ken