Oversampling on dsPIC33EPXXGS50X family
The quantity of bits are those actually present in the result, not the resolution. As such they should be >= the resolution. 

In other words, if the ADC has a resolution of 12 bits, the x2 oversampling gives you 0.5 bit more resolution but to report it you must use 13 bits. 

So the result with 4x oversampling is 'better' in terms of resolution than the 2x oversampling but is presented to the same number of bits.