I'm trying to figure out how to match the following rules...

* Exactly 4 characters (alphanumeric) or zero characters
* Characters can spaces if only used to pad on left
* Can NOT contain just 4 spaces

**Example of acceptable matches:**

```
""
"  43"
" D42"

```

**Example of NON-acceptable matches:**

```
"1 A3"
"1   "
"    "
"A 1 "

```

I have tried using the following regex but it just does not give the expected results.

```
(([\s]*[0-9A-Z]*){4}|)

```