1st - the 4 band limit on the Wiim is likely due to limited processing capacity - i.e.: limted DSP computational power...
The 4 band limit is even worse in that 2 of the 4 bands are "shelf" filters, so only 2 are "peak" filters -- the peak filters are generally used to adjust/reduce frequency peaks or dips, and you need more than 2 to deal with room resonances/peaks below 500 Hz, the "shelving" filters cannot "notch" or reduce peaks at specific frequency ranges, they affect all frequencies equally below (low shelf) or above (high shelf) the selected "transition" frequency.
So for room EQ/correction, you really only have 2 Peak bands to use, and a limited function "Low Shelf" filter to raise or lower the range of frequencies below the selected "transition" frequency.
The "High Shelf" filter is of practically zero use for room correction in the room-dependant frequency region (below 500 Hz).
Reading Wiim's forums, they may update the Parametric EQ further by adding High Pass/Low Pass filter options as an alternative to the High Shelf/Low Shelf filters, but this still only leaves us with 2 Peak Filters.
I invite any correnctions by persons more knowledgable than I on the Parametric EQ topic, but what I've noted above is my understanding of the functionalities of the current 4 Band Parametric EQ of the Wiim Pro and Pro Plus...