Normally we pointed the primary dns and secodary dns to our server dns. there are 3rd dns and 4th dns over there, normally we put it blank. How to use the 3rd and 4th DNS? can we pointed the 3rd dns and 4th dns to another server which has similar content? and once the main server down, it will pointed to 2nd server?
Nope that won't work. DNS won't refresh instantly, and because it will be cached, even if your first host is down some users will still be accessing that host. (I think)
As per the RFC rules, any domain should have at least 2 DNS servers. It is not necessary to have more than 2 if first 2 DNS servers are capable to serving DNS requests. If they are not capable, you may consequently need more than 2 DNS servers. In your case, if your web hosting provider provides the nameserver for 3rd and 4th DNS server then you can put them there. If they do not have more than 2, you need to keep entries for 3rd and 4th blank.
oh ya, I saw before some web hosting server do hv 4 dns. by the way, any solution to have 2 share server, alternative one will up when primary server down?