data = ('a'..'d').to_a
N = data.size

indices = data.map { |_| 0 }

loop do
  puts((0...N).map { |i| data[indices[i]] }.join(''))
  indices[-1] += 1
  looking_index = N - 1
  while indices[looking_index] == N
    indices[looking_index] = 0
    looking_index -= 1
    return if looking_index < 0
    indices[looking_index] += 1
  end
end