The way i'm reading it the scenario you have described is possible. You will have 3 principal instances with a principal database on each that use the 4th server as a mirror instance is that correct?
What you can't do is have 1 principal mirroring a database out to 3 mirror instances, this is only possible with AlwaysOn groups in SQL Server 2012.
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"Ya can't make an omelette without breaking just a few eggs" 😉