Freshers must have a very sound knowledge of Linux fundamentals. They are requested not to cram the commands, but understand the concept. RHCE level of knowledge is must for any fresher to start his/her Linux Administrator career. Give more focus on the first two books of RHCE training. Based on the current feedback, companies are also asking questions related to Bash Shell Scripting, Database and Puppet during interviews. So additional skills of Puppet or Bash Shell scripting will help you getting started, fast.
Linux Admins with 1-2 years of experience are expected to know beyond RHCE. Skills on Linux Virtualization (RHCVA) and Securing Linux Servers (Red Hat Server Hardening) and LDAP (OpenLDAP) are most demanded from experienced Linux Engineers. Knowledge of “Configuration Management Tools” like Puppet will always give an edge.
With Your level of experience you should be trying to position yourself at a “Senior” or “Expert” level of job profile. Sound knowledge of Cloud Infrastructure Management (OpenStack), Cloud Storage , knowledge of Monitoring Tools like Nagios is a must. You should be focusing on complete infrastructure management skills rather than a single server management skills.
With 4+ years of Linux Administration career. Linux skills related to Performance Tuning, Linux HA Clusters, SAN Storage and managing Red Hat Satellite Server is desired. You should focus on getting RHCA Certification to prove your capabilities. It will also help you move to Architect Level in Linux Administration.