diff --git a/2023/04.cpp b/2023/04.cpp
new file mode 100644
index 0000000..e62afc4
--- /dev/null
+++ b/2023/04.cpp
@@ -0,0 +1,110 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include <boost/phoenix.hpp>
+#include <boost/spirit/include/qi.hpp>
+
+#include <doctest.h>
+
+#include <aocpp/Parsing.hpp>
+#include <aocpp/Startup.hpp>
+
+namespace {
+
+namespace qi = boost::spirit::qi;
+namespace phx = boost::phoenix;
+namespace ascii = boost::spirit::ascii;
+
+struct Card
+{
+    unsigned long long id;
+    std::vector<unsigned long long> mine;
+    std::vector<unsigned long long> winners;
+};
+
+struct Grammar : public qi::grammar<std::string::const_iterator, std::vector<Card>(), ascii::space_type> {
+  qi::rule<iterator_type, Card(), ascii::space_type> card;
+  qi::rule<iterator_type, std::vector<Card>(), ascii::space_type> cards;
+
+  Grammar() : base_type{cards} {
+    using namespace qi::labels;
+    cards = *card;
+    card =
+      "Card" >> qi::ulong_long [ phx::bind(&Card::id, _val) = _1] >> ":" >>
+      (*qi::ulong_long) [ phx::bind(&Card::mine, _val) = _1] >> "|" >>
+      (*qi::ulong_long) [ phx::bind(&Card::winners, _val) = _1];
+  }
+};
+
+auto CountWins(std::vector<Card>& cards) -> std::vector<std::size_t>
+{
+  std::vector<std::size_t> result;
+  result.reserve(cards.size());
+
+  for (auto& card : cards)
+  {
+    std::vector<unsigned long long> xs;
+    std::sort(card.mine.begin(), card.mine.end());
+    std::sort(card.winners.begin(), card.winners.end());
+    std::set_intersection(card.mine.begin(), card.mine.end(), card.winners.begin(), card.winners.end(), std::back_inserter(xs));
+    result.push_back(xs.size());
+  }
+
+  return result;
+}
+
+auto Part1(std::vector<std::size_t> const& wins) -> std::uint64_t
+{
+  std::uint64_t result = 0;
+  for (auto const win : wins)
+  {
+    if (win > 0)
+    {
+      result += std::uint64_t{1} << (win - 1);
+    }
+  }
+  return result;
+}
+
+auto Part2(std::vector<std::size_t> const& wins) -> std::size_t
+{
+  std::vector<std::size_t> psums(wins);
+  psums.push_back(0);
+  for (std::size_t i = wins.size(); i --> 0;)
+  {
+    psums[i] = 1 + 2 * psums[i+1] - psums[1 + i + psums[i]];
+  }
+  return psums.front();
+}
+
+} // namespace
+
+TEST_SUITE("2023-04 examples") {
+  TEST_CASE("example") {
+    std::istringstream in {
+R"(Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+)"
+    };
+    auto input = aocpp::ParseGrammar_(Grammar{}, in);
+    auto const wins = CountWins(input);
+    CHECK(Part1(wins) == 13);
+    CHECK(Part2(wins) == 30);
+  }
+}
+
+auto Main(std::istream & in, std::ostream & out) -> void
+{
+  auto input = aocpp::ParseGrammar_(Grammar{}, in);
+  auto const wins = CountWins(input);
+  out << "Part 1: " << Part1(wins) << std::endl;
+  out << "Part 2: " << Part2(wins) << std::endl;
+}
diff --git a/2023/CMakeLists.txt b/2023/CMakeLists.txt
index 9a646ac..bff6a05 100644
--- a/2023/CMakeLists.txt
+++ b/2023/CMakeLists.txt
@@ -1,2 +1,5 @@
 add_executable(2023_02 02.cpp)
 target_link_libraries(2023_02 aocpp Boost::headers)
+
+add_executable(2023_04 04.cpp)
+target_link_libraries(2023_04 aocpp Boost::headers)