The reduced single-digit is used to determine the numerological influence of a telephone number. Here is an example, reducing the number 123-456-7890

1+2+3+4+5+6+7+8+9+0 = 45
                4+5 = 9

The reduced single-digit for 123-456-7890 is the number 9.

The single-digit numbers 8 and 4 are good general business numbers. Other single-digit numbers are good for other types of businesses, such as 3 and 5 for nightclub and travel agency businesses.

For a telephone number used primarily for general business contacts, an 8 or a 4 is good.

In your case, I would suggest the single-digit number 4, because it matches your numerology life path number as calculated from your birth date.