Observations:

• Woof, data validation… kinda tedious. A lot like work, and I do AoC to take a break from work…
• I anticipated having to look up things, so I parsed into a map, but that ended up being entirely unnecessary.
• Thank goodness for pattern matching.

Excerpt:

  def all_fields_valid?(record) do
    Enum.all?(record, fn {field, value} -> valid?(field, value) end)
  end

  def valid?(field, year) when field in ["byr", "eyr", "iyr"] and is_binary(year),
    do: valid?(field, String.to_integer(year))

  def valid?("byr", year) when year in 1920..2002, do: true
  def valid?("iyr", year) when year in 2010..2020, do: true
  def valid?("eyr", year) when year in 2020..2030, do: true
  def valid?("hgt", {cm, "cm"}) when cm in 150..193, do: true
  def valid?("hgt", {inch, "in"}) when inch in 59..76, do: true
  def valid?("hgt", {_, _}), do: false
  def valid?("hgt", height), do: valid?("hgt", Integer.parse(height))

  def valid?("hcl", color), do: Regex.match?(~r/^#[0-9a-f]{6}$/, color)

  def valid?("ecl", color) when color in ["amb", "blu", "brn", "gry", "grn", "hzl", "oth"],
    do: true

  def valid?("pid", pid), do: Regex.match?(~r/^[0-9]{9}$/, pid)
  def valid?("cid", _), do: true
  def valid?(_, _), do: false

A straightforward solution for Part 2:

#!/usr/bin/env elixir

defmodule Validator do

  def valid?({"byr", value}) do
    value
    |> String.to_integer()
    |> Kernel.in(1920..2002)
  end

  def valid?({"iyr", value}) do
    value
    |> String.to_integer()
    |> Kernel.in(2010..2020)
  end

  def valid?({"eyr", value}) do
    value
    |> String.to_integer()
    |> Kernel.in(2020..2030)
  end

  def valid?({"hgt", value}) do
    case String.split(value, ~r/(?<=\d)(?=[a-z])/) do
      [num, unit] ->
        valid_height?(String.to_integer(num), unit)
      _ -> false
    end
  end

  def valid?({"hcl", value}) do
    value =~ ~r/^\#[0-9a-f]{6}$/
  end

  def valid?({"ecl", value}) do
    value in ~w[amb blu brn gry grn hzl oth]
  end

  def valid?({"pid", value}) do
    value =~ ~r/^\d{9}$/
  end

  def valid?({_key, _value}) do
    true
  end

  defp valid_height?(num, "cm"), do: num in (150..193)
  defp valid_height?(num, "in"), do: num in (59..76)
  defp valid_height?(_, _), do: false
end

required_fields = ~w[
  byr
  iyr
  eyr
  hgt
  hcl
  ecl
  pid
]

to_passport = fn(fields)->
  fields
  |> Stream.map(&String.split(&1, ":"))
  |> Stream.map(&List.to_tuple/1)
  |> Map.new()
end

valid? = fn(passport)->
  has_all_required_keys? = Enum.all?(required_fields, &Map.has_key?(passport, &1))
  all_values_are_valid? = Enum.all?(passport, &Validator.valid?/1)
  has_all_required_keys? and all_values_are_valid?
end

"day4.txt"
|> File.read!()
|> String.split("\n\n")
|> Stream.map(&String.split/1)
|> Enum.map(to_passport)
|> Enum.count(valid?)
|> IO.puts()

As this is typical day to day work I just imported Ecto in my solution