$4 may be high/low depending on the nitche, you should focus on building traffic and optimizing ad placement
it depends on your niche. Health sites, Debt, Loans related sites will give you max $$$ on the CPM/CPC basis.
depending on the genre. Im averaging about $6. But some sites can be as low as $2. But then you can get really weirdly high cpm. So it constantly fluctuates.
I agree with you, that those niches are great for Adsense earning, but it`s not so easy to get first positions in those nices in search engines...