Two things:
1) A 52" high wall means 4' of water is only 4" below the wall, so it's not 4.5'--Sorry, Watermom!
2) If your pH is measuring 6.8-7.2, then I suspect you are using strips and they are nearly IMPOSSIBLE to read correctly for pH. You need at least an inexpensive OTO/pH test kit to get a more accurate reading of your pH before you begin adding Borax.

Carl