Roman numeral kata in erlang. Wondering if there is a better/more idiomatic way to approach the problem.

roman_numerals.erl

-module(kata).
-include_lib("eunit/include/eunit.hrl").

to_roman(Arabic) when Arabic >=  50 -> string:concat("L",  to_roman(Arabic - 50));
to_roman(Arabic) when Arabic >=  10 -> string:concat("X",  to_roman(Arabic - 10));
to_roman(Arabic) when Arabic >=  5  -> string:concat("V",  to_roman(Arabic - 5));
to_roman(Arabic) when Arabic =:= 4  -> string:concat("IV", to_roman(Arabic - 4));
to_roman(Arabic) when Arabic >=  1  -> string:concat("I",  to_roman(Arabic - 1));
to_roman(_) -> "".

it_transform_the_number_1_to_roman_numeral_I_test() ->
  ?assertEqual("I", to_roman(1)).

it_transform_the_number_2_to_roman_numeral_II_test() ->
  ?assertEqual("II", to_roman(2)).

it_transform_the_number_3_to_roman_numeral_III_test() ->
  ?assertEqual("III", to_roman(3)).

it_transform_the_number_4_to_roman_numeral_IV_test() ->
  ?assertEqual("IV", to_roman(4)).

it_transform_the_number_5_to_roman_numeral_V_test() ->
  ?assertEqual("V", to_roman(5)).

it_transform_the_number_7_to_roman_numeral_VII_test() ->
  ?assertEqual("VII", to_roman(7)).

it_transforms_the_number_10_to_roman_numeral_X_test() ->
  ?assertEqual("X", to_roman(10)).

it_transforms_the_number_37_to_roman_numeral_XXXVII_test() ->
  ?assertEqual("XXXVII", to_roman(37)).

it_transforms_the_number_57_to_roman_numeral_LVII_test() ->
  ?assertEqual("LVII", to_roman(57)).

There are missing cases (9, 40s etc...) but it's just to get the idea...

Made a little improvement to the code:

to_roman(0)      -> "";
to_roman(Arabic) ->
  {Equivalence, Value} = closest_equivalence_to(Arabic),
  Equivalence ++ to_roman(Arabic - Value).

closest_equivalence_to(Number) when Number >= 10 -> {"X",  10};
closest_equivalence_to(Number) when Number == 9  -> {"IX", 9};
closest_equivalence_to(Number) when Number >= 5  -> {"V",  5};
closest_equivalence_to(Number) when Number == 4  -> {"IV", 4};
closest_equivalence_to(Number) when Number >= 1  -> {"I",  1}.