It's the D7100 i have lol yes i shoot raw by default.
This was the first time that i shot properly in low light.
Active d lighting and noise reduction are for jpegs, not raw. So before you upgrade you should perhaps learn more about the camera you own. Active d-lighting might actually make things worse by under exposing in order to preserve highlights, increasing noise when you balance exposure.
And when you say the noise performance is poor, what are you comparing it to? ISO 4000 is extremely high, are you sure you are not c fused and should be shooting in ISO 400?
What is your post processing like, are you using any nise reduction plugins for LR etc? Something like NoiseNinja will help a lot.
Secondly how are you evaluating the noise, are you zooming into 100% and examining a shadow, or printing a nice 8x10 for your living room and viewing it from 3ft?
As to going FF, you will gain just over 1 stop. ISO 800 will look much like ISO 400 on your d7100, ISO 6400 on the D600 will look like ISO 3200 on your D7100. Depends if you really find this useful. Fr some situations that is a huge benefit, for many it is a small improvement.
And if you are frequently shooting in the dark then a tripod and a flash or 2 will help tremendously under most situations (tripod for static scenes, flashes for close work like people). If you do something like indoor sports or shooting a wedding from the back of a pitch black church then a FF body will help (providing you don't compromise on glass).