If you have four disks and you're mirroring them, unless you're talking about a 4-way mirror (triply redundant), you have to have some RAID0 in there somewhere, so it's either going to be RAID 0+1 or RAID 1+0. With 4 disks, it probably doesn't make a lot of difference.

Well, I suppose you could just have a mirrored set of pairs of concatenated disks, but concatenation isn't common any more. If you are talking about that, then, yes, adding the stripe in there is going to help performance.