I realize this is a bit old, but some pointers if you're still working on it:
don't add the /replication to your masterURL tag
Getting a 404 for mymaster:myport/solr/mycorename is normal.
If you have only one core, chances are you can set the master URL to just mymaster:myport/solr/
This is a pretty good reference as well: https://cwiki.apache.org/confluence/display/solr/Index+Replication