Question about Mac Pro RAID

I've got a Mac Pro that has four SATA drives installed in it, and I've got two of those drives in a RAID-1 mirror, using just the onboard SATA ports and software RAID (no fancy external RAID card or anything). Those two drives are 1TB. I want to replace them with 4TB drives. If I replace one drive at a time, let them each rebuild, afterward will OS X allow me to re-size the partition to take up the full 4TB? Or, can you only do that by wiping out the disks?

I know, "why don't you just back it up somewhere first and restore it" -- long story short, every means I have to do that is going to take forever to move that much data, and I'm only going to have a few hours in which to do this.

Or, any better ideas for this?

I understand it's not ideal but copying to an external drive is going to take forever (that's what we tried to do on the last computer we did this on).

All four hard drive bays are being used in the computer. Two of them are for OS and other, so, I can't really take them out to temporarily put another RAID array in. That's why I'm hoping to just hot-swap these one at a time, let it rebuild (which won't take long over SATA), and then expand the volume. I'm just looking to know if that's technically possible. I'm able to do this on my Synology box at home, but that's a much more advanced RAID device.