building a RAID enclosure.. Is this config possible?

[jdl8422]

I've been searching the internet for a 4 bay enclosure to to build a backup solution and possibly additional storage. I basically want a Drobo but dont want to pay 500 just for the enclosure and no drives. I want to use it as an external HD for time machine on my Mac. I had a friend offer me a PC enclosure, Is is possible to use this and install a RAID controller and and use it as a big external HD? if not, what do I need to build my own external HD with multiple bays. Thanks in advance.