Not an answer, but why not do all of that in the query (I assume PostgreSQL)?

from s in Supplier,
  where: s.company_id == ^id,
  order_by: [desc: :last_modified],
  group_by: s.supplier_id,
  select: fragment("array_agg(?)", s)

About your question, use Enum.flat_map/2 as your Enum.map/2 will return list of lists and in Enum.sort_by/3 you expect list of maps.

In SQL:2003 it could be written as:

SELECT *
FROM (
  SELECT
    s.*,
    dense_rank() over (PARTITION BY s.supplier_id ORDER BY s.last_modified DESC) rank
  FROM suppliers s
  ORDER BY s.last_modified DESC) q
WHERE rank < 5
ORDER BY q.last_modified DESC

Unfortunately Ecto AFAIK still do not allow to have select from subquery so we need to “hack around” by using JOIN:

SELECT s.*
FROM suppliers s
INNER JOIN (
  SELECT
    id,
    dense_rank() over (PARTITION BY supplier_id ORDER BY last_modified DESC) rank
  FROM suppliers) q
  ON q.id = s.id
WHERE q.rank < 5
ORDER BY s.last_modified DESC

This one should be possible to express in Ecto syntax:

from s in Supplier,
  inner_join: q in subquery(from ns in Supplier,
                              select: %{
                                id: ns.id,
                                rank:
                                  row_number()
                                  |> over(partition_by: :supplier_id, order_by: {:desc, :last_modified})
                              }),
  on: s.id == q.id,
  where: q.rank < 5, # row_number is 1-based
  order_by: {:desc, :last_modified}

However this is all based on the assumption that the T-SQL supports window functions in SQL:2003 compatible way or that Ecto can translate above query to syntax used by MS SQL.

These are 100% identical. IIRC once one was implemented using another. There is no strong preference on one over another AFAIK but with time I personally prefer from macro as this do not force me to repeat the match over and over again. Use what you want TBH. Additional advantage of from for me is that it encourages people to write whole query in place as I found the “building” quite confusing (but that was pre-named joins, so nowadays it is probably nicer).

So answer is no, pipe and from are completely equivalent solutions and you can even mix them as you please.