Say there are 4 nodes in cassandra, is it possible that we can send insert/delete/update queries to any of the nodes?
Yes. Using the default partitioner, its designed to connect to any nodes.
Do all the data stores in other nodes need to be runnin on port 9160 as there is not a way to specify port in the list of seeds
Yes. There are more ports than just '9160' to consider. Gossip, Storage, UDP, etc. So as long as the other nodes have similar configs, just setting the IP's in the seed section is good enough.