Another vote to check out the Kahrs, you're not going to find anything much smaller/flatter/lighter.
However, if the Kahrs are too small/light/punishing take a look at the SIG P250 subcompacts. They're very easy to maintain, modular so you can swap grip modules/slides/calibers whenever you like, have a light/smooth DAO-style trigger and are very affordable.
Tomac
These aren't 40's, but counter-clockwise from top right are P250compact, P250subcompact, Kahr P380 and Kahr CM9:
