I have four domain names, but I will have only one site. All the domains will point to this one site. I have read that Google will treat this as duplicate content, and rate that as a bad practice or a black hat move to trick the SEO.
I could do the .htaccess 301 trick, but will this work with all the domains pointing to the same DNS?
I am sure I am not the only one out there with more than one domain!
If you point the domain name to the domain name you want to use it won't be a problem.
For example, redirect www.example.net to www.example.com.
The search engines will typically leave you alone on that.
Another (more common issue) is when someone has example.com and www.example.com pointing to the same place. The solution for that would be something like this:
RewriteCond %{HTTP_HOST} !^www\.example\.com
RewriteRule (.*) http://www.example.com/$1 [R=301,L]
You should be able to take the above example and modify it to fit your needs.
