whats the difference between ns4 and ns2 soap client

whats the difference between ns4 and ns2 soap client

I have a code which I am trying to understand..I see the following in the
code..whats the difference between ns4 and ns2?where can i GET more info
on this?
request = self.search_soapclient.factory.create('ns4:SearchRequest')
subquery =
self.search_soapclient.factory.create('ns2:SubQueryCriteriaEntity')