No, cane toads are most likely not in Oklahoma.
I also might add that 4 inches would be very small for a cane toad; they average 6-9 inches long with 12 inches not uncommon.
Just the other day I caught a large Fowler's toad that was likely around the 4-inch mark. American toads also reach 4 inches quite frequently.
So, in summary, no, not a cane toad, just a big American or Fowler's toad.