Reviewing the last post, I had a thought. I had said that the reason for using the singular al chet in the first three sections was that the single chet of rebellious disobedience was referred to. Any chet committed, no matter what the motivation, instantiates the general concept of rebellion toward, kevayachol, the Al-mighty. This represents an all-encompassing chet; and it is the subject of confession in the first three groupings.
However, it occurs to me now that there is an alternative way of viewing the matter. Drawing on the same basic ideas, we might explain the use of the singular in the first three sections in the following way. In the these sections, the focus is not on the different aveiros considered from the perspective of their defining prohibitions. It is, rather, on them considered from the perspective of the motivation (techunos hanefesh) leading up to them. When regarded in this way, acts of sin are not subsumed under universal classifications, like: eating this or doing that. Each act is particular and unique. It is simply and purely ill-motivated, ill-begotten. (It is rebellion.) It is, you might say, sui generis: an instance unto itself. So it is singularly formulated as a chet, meaning an individual instance. In the fourth section, on the other hand, chata-im are, as noted, delineated by the obligations they incur. What obligation a given perpetration of a chet incurs depends on the prohibition – the category – under which it is subsumed. Moreover, each type of obligation is incurred by any of multiple such prohibition-categories (e.g., different types of aveira obligate the same karban). Accordingly, the plural form – al-chata-im – is used, to denote these various aveira types.