Growing a LVM volume with striping
Solution 1:
Do the previously existing blocks get re-organized?
Nope.
What you would get is a LV that is a concatenation of two stripe sets, one 3 disks and one 4 disks.
However, if you going to extend and LV with stripes then the maximum size of the extension is N * the number of free extents on the PV with the fewest free extents where N is the number of stripes.
In your case the three current disks are full. Thus you cannot use striping at all.