Tweening is actually pretty easy, you can do it without the TweenMax class. (Note this is untested code; I just wrote it up. May have compile errors.)

Code:
import flash.utils.getTimer;

scoreStart:Number = 0;
scoreCurrent:Number = 0;
scoreGoal:Number = 0;
startTime:Number = 0;
setScore(4000);

// ENTER_FRAME event handler
function scoreInterpolate(event:Event)
{
	var currentTime:Number = getTimer();
	var durationTime:Number = 3000;
	var t:Number = (currentTime - startTime) / durationTime;
	if (t < 1)
	{
		// Not finished counting up; interpolate value
		scoreCurrent = linearInterpolate(t, scoreStart, scoreGoal);
	}
	else
	{
		// At or exceeded the maximum time
		//  Set the score to the target and terminate the event
		scoreCurrent = scoreGoal;
		removeEventListener(Event.ENTER_FRAME, scoreInterpolate)
	}
	trace(scoreCurrent);
}

// Performs a linear interpolation between two values
function linearInterpolate(t:Number, minVal:Number, maxVal:Number) : Number
{
	return minVal + t * (maxVal - minVal);
}

// Invokes an ENTER_FRAME event handler that will count up scoreCurrent to the provided goal
function setScore(newScore:Number)
{
	scoreStart = scoreCurrent;
	scoreGoal = newScore;
	startTime = getTimer();
	addEventListener(Event.ENTER_FRAME, scoreInterpolate);
}
This will perform a linear interpolation to count up. (ie the amount will grow by the same amount each frame) You could easily provide a function that will do exponential growth or anything else you wanted. (See here.)