Excel Template

I am trying to create a macro that will run when a worksheet is opened and automatically insert a row every fourth row. The data is queried in a database and exported to Excel, and I need it separated. Some worksheets may only have 10 lines of data, but others may have 200. How do I tell it to do this? I am a newbie at VB, but that looks like the place to do it. Thanks for any help anyone can provide.