Need help writing an int to roman numeral converter using only if, if else, and switch statements

Need help writing an int to roman numeral converter using only if, if
else, and switch statements

Here is the code:
public String ToRoman(int number) {
if ((number < 1 || (number > 3999)))
if (number >= 1000) return "M" + ToRoman(number - 1000);
if (number >= 900) return "CM" + ToRoman(number - 900);
if (number >= 500) return "D" + ToRoman(number - 500);
if (number >= 400) return "CD" + ToRoman(number - 400);
if (number >= 100) return "C" + ToRoman(number - 100);
if (number >= 90) return "XC" + ToRoman(number - 90);
if (number >= 50) return "L" + ToRoman(number - 50);
if (number >= 40) return "XL" + ToRoman(number - 40);
if (number >= 10) return "X" + ToRoman(number - 10);
if (number >= 9) return "IX" + ToRoman(number - 9);
if (number >= 5) return "V" + ToRoman(number - 5);
if (number >= 4) return "IV" + ToRoman(number - 4);
if (number >= 1) return "I" + ToRoman(number - 1);
Scanner myKeyboard = new Scanner (System.in);
System.out.println("Enter the integer: ");
number = myKeyboard.nextInt();
myKeyboard.close();
} }
The problem im having is that i get an error saying "The method must
return a result type string".