[flexcoders] Disable button if field is empty

[mr_delphi_developer]

I have this input field and button.

<mx:TextInput id="team_pin" maxChars="4" displayAsPassword="true"
editable="true" enabled="true" width="55" />

   <mx:Button label="Get Roster" click="empRO.loadAll.send();"
enabled="false" id="GetRosterBtn"/>

I don't want the button enabled until the team_pin has 4 characters.

I've tried enabled="{team_pin.length < 4}" but it says 
"Invalid value: It must be one of true, false"

How would I do that?