The internal ADC of TI 28xx is crap. It has tremendously inacurate internal reference and high zero offset. With 12 bits available, the true accuracy is about 8 bits unless you have the ADC calibrated.
There is no need for 4 layer board with 28xx. The layout can be done on two layers just fine. Been there, done that.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant