I just wanted to try Universal Forwarder agent of Splunk v5 on my test Active Directory domain controller which is running on Windows 2008.
The service accounts privileges were Ok and I applied the whole procedure. By the way the service account had Domain Admin privileges but each time I tried to install the agent, I had the same error message.
"splunk installer was unable to start splunk services exit code '4'"
The resolution is weird, pointless but simple.
Try to type the service account as "account@domain" instead of "domain\account".

