service tax

If 4 services are running in a same roof, so what is the procedure for service tax registration.  Out of 4, 1 service is already registered in service tax. Service tax number is taken as voluntary, means turnover is below the limit required. My question is that should we make amendment in existing service or separate registration is required for each service. It's a new setup; currently we are not charging service tax. There would be any penalty for that? Please advice. [Thanks in advance]