MongoDB only three of four shards being used
There are a few confusing pieces here, first you list this piece of sh.status():

{  "_id" : "test",  "partitioned" : false,  "primary" : "d" }
{  "_id" : "test_collection",  "partitioned" : false,  "primary" : "d" }


That means that there is a test database, a test_collection database, not collections.  It also indicates that neither of them are sharded - that is what "partitioned" : false means.  

You then show some output from test_cluster.test_cluster_collection.stats() but that is not one of the databases from the sh.status command.  It is a collection from the test_cluster database, and there is no indication of how that is sharded, though it would be listed in sh.status().  Basically the two pieces of information have nothing to do with each other.

Your for loop will also be inserting into the test_sharded_collection as a sub-collection of test_cluster.  In other words, the full namespace you are inserting into is test_cluster.test_cluster.test_sharded_collection, and I suspect that is not sharded which is why you are likely not seeing the chunks increase.  What I suspect you intended to do was insert into test_cluster.test_sharded_collection.  Try this for your inserts instead:

use test_cluster;
for( var i=0; i &lt; 20000; i++ ){db.test_sharded_collection.insert( { x:i,y:3,z:"test                       test" } )};


The full output of sh.status(), and specifically the section about the test_sharded_collection would help me confirm my suspicions.