diff --git a/2025/ruby/day_04.rb b/2025/ruby/day_04.rb
new file mode 100644
index 0000000..da8ce71
--- /dev/null
+++ b/2025/ruby/day_04.rb
@@ -0,0 +1,44 @@
+NEIGHBORS = [
+  [-1,-1], [-1,0], [-1,1],
+  [ 0,-1],         [ 0,1],
+  [ 1,-1], [ 1,0], [1 ,1],
+]
+
+Grid = Data.define(:rolls) do
+  def size = rolls.size
+
+  def accessible
+    rolls.select {|(y,x)|
+      NEIGHBORS.count {|(dy,dx)|
+        rolls.include?([y+dy, x+dx])
+      } < 4
+    }
+  end
+
+  def removed
+    return enum_for(__method__) unless block_given?
+
+    grid = self
+    loop do
+      yield grid
+      grid = Grid.new(grid.rolls - grid.accessible)
+    end
+  end
+end
+
+grid = Grid.new(
+  DATA.read
+    .lines(chomp: true)
+    .each.with_index.inject(Set.new) do |rolls, (line, y)|
+      rolls.merge(
+        line.chars.each.with_index
+          .select {|roll,_| roll == ?@ }
+          .map { [y, _2] }
+      )
+    end
+)
+
+chunk = grid.removed.slice_when { _1.size == _2.size }.next
+pp chunk.first.size - chunk.last.size
+
+__END__