Theoretically, Hammer, that should be ok. The old "rule" for retaining is ......... thickness of wall should be at least 1/4 of the height.
100mm brickwork isn't going to look very substantial though......... and not very appealing without a decent capping of some sort.
If you really want to impress him ............... put up one course (9" high) of concrete blocks around the pond........... then face it with 100mm brick, and finish with a capping of brick on edge (bricks laid side by side on edge. (13" finished height.)
Put in more courses to increase height if you wish.
If you do go for 100mm brick alone ............... put this in a couple of the horizontal beds.......... increases brickwork strength tenfold...........
http://www.screwfix.c...oil-112mm-x-20m/69577