Your requirements are less then what Splunk used as their reference server, so starting with their reference server would be a good start. 4-10GB/day is less then what many customers generate.
Your best bet is to purchase hardware which has around the same amount of CPU and memory as the server here, and then focus on getting the fastest disks possible (high RPM, high IOPS) which can meet your storage requirements (How many months do you want to retain the data?). Storage IOPS really, really matter.
A good place to start is here:
http://docs.splunk.com/Documentation/Splunk/5.0.2/Installation/Referencehardware
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