Subsets(4), every subset drawn
Sixteen rows, in the binary (Gray-free) order the kernel unranks. Sort by Length to group by size; filter Length(_) == 2 for the six pairs -- the count goes exact at once, since sixteen rows scan in a millisecond. The Sum column is any expression over the row, not just a head.
source
CollectionTable(
Subsets(4),
(Columns, "Length, Sum"),
(Glyph, "subset"),
(PageSize, 10),
)notebook cells, the CLI