diff --git a/README.md b/README.md
index bb10e86..49d4b90 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ I'm also adding notes that may be useful if you're learning Ruby.
 Notes for solving:
 * [2015, complete](year_2015.md)
 * [2016, up to day 08](year_2016.md)
-* [2023, up to day 03](year_2023.md)
+* [2023, up to day 04](year_2023.md)
 
 # How to use
 Install dependencies with `bundle install`. If you are in a hurry, just install [RSpec](https://github.com/rspec/rspec-metagem) with `gem install rspec` and run `rspec` in the root directory.
diff --git a/spec/year_2015/day_04_input b/spec/year_2023/day_04_input
similarity index 100%
rename from spec/year_2015/day_04_input
rename to spec/year_2023/day_04_input
diff --git a/spec/year_2015/day_04_sample_one b/spec/year_2023/day_04_sample_one
similarity index 100%
rename from spec/year_2015/day_04_sample_one
rename to spec/year_2023/day_04_sample_one
diff --git a/spec/year_2015/day_04_sample_two b/spec/year_2023/day_04_sample_two
similarity index 100%
rename from spec/year_2015/day_04_sample_two
rename to spec/year_2023/day_04_sample_two
diff --git a/spec/year_2015/day_04_spec.rb b/spec/year_2023/day_04_spec.rb
similarity index 83%
rename from spec/year_2015/day_04_spec.rb
rename to spec/year_2023/day_04_spec.rb
index 0af03d0..1987127 100644
--- a/spec/year_2015/day_04_spec.rb
+++ b/spec/year_2023/day_04_spec.rb
@@ -1,9 +1,9 @@
-require 'year_2015/day_04'
+require 'year_2023/day_04'
 
-describe Year2015::Day04 do
+describe Year2023::Day04 do
   context 'when Part 1' do
     subject(:sample_one) do
-      described_class.new(File.read('spec/year_2015/day_04_sample_one'), true)
+      described_class.new(File.read('spec/year_2023/day_04_sample_one'), true)
     end
 
     it 'finds winning numbers' do
@@ -25,7 +25,7 @@
 
   context 'when Part 2' do
     subject(:sample_two) do
-      described_class.new(File.read('spec/year_2015/day_04_sample_one'))
+      described_class.new(File.read('spec/year_2023/day_04_sample_one'))
     end
 
     it 'distributes cards' do
@@ -41,7 +41,7 @@
 
   context 'when Results' do
     subject(:input_data) do
-      File.read('spec/year_2015/day_04_input')
+      File.read('spec/year_2023/day_04_input')
     end
 
     it 'correctly answers part 1' do
diff --git a/year_2023.md b/year_2023.md
index 2b8ec15..9ab406a 100644
--- a/year_2023.md
+++ b/year_2023.md
@@ -77,4 +77,24 @@ I'm clearly not good enough with spatial representations (something about off-by
 
 Maybe I'll revisit this one with a better solution.
 
-Also, thanks to the kind people or [/r/adventofcode](https://www.reddit.com/r/adventofcode/comments/189q9wv/2023_day_3_another_sample_grid_to_use/) for the sample grids they provided, they were a great help in debugging the first part properly.
\ No newline at end of file
+Also, thanks to the kind people or [/r/adventofcode](https://www.reddit.com/r/adventofcode/comments/189q9wv/2023_day_3_another_sample_grid_to_use/) for the sample grids they provided, they were a great help in debugging the first part properly.
+
+## Day 04: Scratchcards
+
+```
+Year2015::Day04
+  Part 1
+    finds winning numbers
+    calculates points
+    gives a final result
+  Part 2
+    distributes cards
+    gives a final result
+  Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+Part one is pretty easy. One useful method is the [`#&`](https://ruby-doc.org/core-3.0.1/Array.html#method-i-26) operator that takes two arrays and returns a new one with only the matching contents. The syntax is clearly inspired by [bit masking](https://en.wikipedia.org/wiki/Mask_(computing)), which is a subject you should look into if you've never heard of it.
+
+Part two is a bit more annoying and requires to do a two-pass, but nothing really improbable.
diff --git a/year_2015/day_04.rb b/year_2023/day_04.rb
similarity index 98%
rename from year_2015/day_04.rb
rename to year_2023/day_04.rb
index 0add241..e8da477 100644
--- a/year_2015/day_04.rb
+++ b/year_2023/day_04.rb
@@ -1,4 +1,4 @@
-class Year2015
+class Year2023
   class Day04
     attr_reader :version, :lines