Modern Roman Numeral generator - am I missing a way to make this simpler? I know I could use a case statement and tighten things up that way, but logically, am I missing something?

modern_roman.rb

def roman input
  if input < 0 || input >= 10000
    'Out of bounds'
  elsif input >= 1000 && input < 10000
    'M' * (input / 1000) + roman(input % 1000)
  elsif input >=900 && input < 1000
    'CM' + roman(input-900)
  elsif input >= 500 && input < 900
    'D' + roman(input-500)
  elsif input >= 400 && input < 500
    'CD' + roman(input-400)
  elsif input >= 100 && input < 400
    'C' * (input / 100) + roman(input % 100)
  elsif input >= 90 && input < 100
    'XC' + roman(input-90)
  elsif input >= 50 && input < 90
    'L' + roman(input-50)
  elsif input >= 40 && input < 50
    'XL' + roman(input-40)
  elsif input >= 10 && input < 40
    'X' * (input/10) + roman(input % 10)
  elsif input == 9
    'IX'
  elsif input >= 5 && input < 10
    'V' + roman(input-5)
  elsif input == 4
    'IV'
  elsif input >= 1 && input < 4
    'I' * input
  else
    ''
  end
end

my_entry = ""

while my_entry != "end"
	print "Integer to see as Roman numeral ('end' to quit): "
	my_entry = gets.chomp
	break if my_entry == "end"
	puts "#{my_entry} in roman numerals is: #{roman(my_entry.to_i)}"
end