Terry,I think it would depend on the distance of your subject.The general opinion re converters seems to be that the image is sharper if the converter is not used.If your 50D has the same sensor as the 40D,then if your image is sharp,a crop will not spoil your picture.The main subject will be just as sharp when cropped,in fact is usually sharper.
You really can crop around a small bird,and ,again,if your subject is sharp,no camera shake etc,good light,you will have a good enlargement.Better than if you had used a 1x4.
But hopefully someone with more knowledge than myself will perhaps offer you more info.You can take sharp shots with a 1x4,but sometimes they need to be closer,as opposed to a greater distance away,but if they are reasonably close ,then you do not need the 1x4.Hope this makes sense.