RTOS for Cortex M4 with 802.15.4/6LoWPAN stack
If you are not married to that specific processor (or are savvy enough to copy/paste the code out anyway), I frequently use Freescale MCUs with CodeWarrior and Processor Expert. PEX includes a number of components including FreeRTOS, MQX, FAT, etc. Additional components can be downloaded and, in the end, it is just a GUI-based code-generator so, as suggested, you could copy/paste the resulting C code into your project.

Edit:

-MQX includes IP stack

-FNET

Large pre-compiled package of many useful components:
http://sourceforge.net/projects/mcuoneclipse/files/PEx%20Components/