to say or argue that 4 megapixels "isn't enough" or is a drawback is a fallacy. What isn't it enough for? Clearly it's been demonstrated in major publications that 4 megapixels is plenty.
4 megapixels isn't enough for a large 300DPI print. It's mathematical.
In the old days you wouldn't print digital pictures at 300DPI because you just couldn't, thus there wasn't a demand for such thing. With new cameras with higher resolution you can.
Is this capability really useful? That's what the purchaser needs to decide for him or herself.
Ultimately your argument is invalid. People using black and white film cameras got their pictures printed on magazines and major publications. If it was good enough for them then why isn't it good enough for us now?
Obviously things have been changing since the early days of film photography and the vast majority of the phototgraphy market is demanding color pictures and high resolution images.
Again, you could buy a film camera and take black and white pictures if you want. It's up to the buyer to decide what they do and don't want.
My point is that most people need something more than 4MP to make business out of photography. Clients demand high resolution photos, microstock sites pay more for higher resolution images and many magazines ask for 300DPI files. (with 4mp that means your pictures will only be 8x5cm).
To sum up: Times have changed.