What is the meaning of "dhammā" in the context of the four satipaṭṭhāna? I've been struggling with this one for quite a while now. The fourth foundation of mindfulness is called 'dhammānupassana' - vision in regards to the dhammas. The old translation I was given when I started meditating was "mind objects", which I'm pretty sure is not at all correct in this context, since it doesn't get at the distinction between the fourth set and the first three.

The phrase "mind object" is a translation of "dhammarammana". According to Nina Van Gorkom, 


  Dhammarammana comprises all objects which are not included in the first five classes. These can be experienced only through the mind-door.
  
  http://www.budsas.org/ebud/nina-abhidhamma/nina-abhi-16.htm


So, "mind object" is a specific subset of reality in a specific context. The first five indriya and the first three elements that comprise rupakkhandha are not a part of this subset, and yet they appear in the section on dhammanupassana.

The commentary says relating the the fourth satipatthana, 


  and now in order to speak of even the laying hold of the aggregates of perception and formations, he said "And, how, o bhikkhus," and so forth." 


So the emphasis is supposed to be on these two aggregates (even though the five aggregates themselves are under this heading).

In Thai, the meaning is given as "that which holds (dhareti) the meditator from falling into evil", which is a definition of the truths (dhammas) taught by the Buddha. That seems appropriate, but it is still curious that there are only a limited number of subsets of the Buddha's teaching in the satipaṭṭhāna sutta, certainly not all of the Buddha's teaching.

Does anyone have a proper explanation as to why the fourth set is called "dhammas"?
I think the following perspective on all four satipatthanas will help in understanding the  meaning of the fourth one. The four satipatthanas are supposed to counter the four vipallasas (perversions) which are: permanence, happiness, self, and beauty. So, mindfulness of the body counters the beauty perversion; mindfulness of feelings (and the fact that all vedanas are included in dukkha) counters the happiness perversion; mindfulness of mind (which is anicca) counters the permanence perversion; and, finally, mindfulness of the dhammas counters the self (atta) perversion. Thus, one reviews dhammas to realize anatta. To view four perversions would be inappropriate attention (ayoniso-manasikara), but the four satipatthanas, would be training in appropriate attention (yoniso-manasikara).

So, overall, I think the meaning of dhamma in this context is exactly what they are: analytical properties of experience, ultimate phenomena, and everything else that apply to dhammas (mind objects, matikas, dhamma lists, etc).