How many RAID drives can one use in a MAC Pro?

I know the question sounds simple. I have a Mac Pro with four matched drives (purchased specifically to do RAID). I want speed over everything else with these drives, and use a automated backup every night to create an external bootable backup, and once a week to do another, so I am comfortable with using all four internals for RAID. Also I have had my Mac for several months and have tried several configurations. I also used software raid in my old windows workstation (2 drives), Here is the reason for my question. In the Mac, as I have gone beyond 2 drives, I have noticed lots of speed, but issues with slowdowns when dealing with drives out of the raid set, 2 drives is fast and very stable, and "normal". 3 drives is noticably faster but has problems communicating with the single, fourth drive and external drives. 4 drives were tried early on and had problems, but that was long enough ago that I am not sure what was going on. My question: I assume that the four drives in the Mac operate on two SATA busses. Is there any reason, other than perhaps a my own hardware or install problem (ie, not a Mac limitation), that should prevent me from using all 4 drives in a single Mac disk utility software raid? has anyone done this? I want to consider this one more time before i consider the machine to be in its "final setup". Side point: Doing all the installs I have done to experiment with these questions has made me very comfortable with the Mac, and I am probably much further along in my "PC to Mac" transition that I ever would have been otherwise. Oh, on that line, BOOT CAMP is not being used, so that does not factor into the question. I certainly appreciate the consideration anyone would give to this post.

Regards, Arthur in Baton Rouge