Google will keep hitting the 404 to see if it has been changed to a live page.
Does Google Still Crawl 404 Not Found Pages?
I have the main domain name as Dublinnites.ie and two other domans that redirect to the main one, Dublinnites.com and Dublinnights.ie, google has indexed the site for the Dublinnites.com domain, when i click on say www.dublinnites.com/disclaimer.php it will show up a 404 error, will these pages just fall out of google when google crawls realises that the two domains are just redirecting??
why don't you use domain pointers, this way will not show 404, unless you have another reason why the other domains are registred as having different websites?
customise the 404 page as well or do a 301 redirect
A 404 page should return a 404 HTTP response. If it doesn't you will run into issues.
I'd normally use a custom 404 page with a link to the main site map or similar.
That way it gives the correct response code and if a human or spider lands on it it's not a "dead end"