Number Field - Limit Four Characters - Validation Rules

You have three key issues:

1. You need to use < instead of <>.
2. You cannot call LEN on a Number.
3. You need to use OR instead of AND.

1. Less Than <> Not Equal

If you want to set a minimum length, you need to use the < operator. You need less than to validate the length doesn't go below your threshold rather than not equal.

Use > for greater than.

Use <> for not equal.

Use < for less than.

2. Number <> Text

You are trying to treat numbers as text.

The correct way to check if a number is blank:

ISNULL(NumberField__c)

The correct way to check if a numeric text field is blank:

ISBLANK(NumericTextField__c)

The correct way to check if the length of a number is less than 4:

NumberField__c < 1000

The correct way to check if the length of a numeric text field is less than 4:

LEN(NumericTextField__c) < 4

3. OR <> AND

If you use the checks on length described in #2, it is impossible for both criteria to be true at the same time. The following formula should work for a number field:

OR(
    ISNULL(NumberField__c),
    NumberField__c < 1000
)

The following should work for numeric text:

OR(
    ISBLANK(NumericTextField__c),
    LEN(NumericTextField__c) < 4
)

The above formula for number results in:

record.NumberField__c = null;    // validation error
record.NumberField__c = 0;       // validation error
record.NumberField__c = 10;      // validation error
record.NumberField__c = 100;     // validation error
record.NumberField__c = 1000;    // valid
record.NumberField__c = 10000;   // valid
record.NumberField__c = 100000;  // valid
record.NumberField__c = 1000000; // valid

The above formula for numeric text results in:

record.NumericTextField__c = null;      // validation error
record.NumericTextField__c = '';        // validation error
record.NumericTextField__c = '10';      // validation error
record.NumericTextField__c = '100';     // validation error
record.NumericTextField__c = '001';     // validation error
record.NumericTextField__c = '0001';    // valid
record.NumericTextField__c = '1000';    // valid
record.NumericTextField__c = '10000';   // valid

1) Try converting number to a text using TEXT() function and find the length using LEN()

2) If you want to enforce say minimum 4 characters, also check if the numberfield is not blank you need to use OR instead of AND. When you use AND both conditions need to be true, in this case it cannot be blank as well as be <> 4 characters. If you check numberfield length is not equal to 4 anything above or less than 4 will fail the validation.

OR(
  ISBLANK(NumberField__c),
  LEN(TEXT(NumberField__c)) <> 4
  )