I'd say gator gar if you're not worried too much about the koi... miss a feeding and you might miss a few koi. Pacu are a choice but again might cause an issue with the koi, arapaima is probably a bit too large for that pond size as well. Perhaps some catfish would work - RCT, TSN, Achara Cats, any hybrid of those three, and/or niger/ripsaw catfish would be cool, and would make for a nice wet pet as well. Arowana might work, but you'd have to be willing to risk keeping it in an open pond. Is the pond in ground or no?
That should provide you a good starting point. I'm not sure I would go with any gar species other than an alligator gar, just because of the fact that there could be issues with the koi damaging the spines of the gar during feeding time - accidents happen and gar tend to be fragile.One other fish to try would be a clown knifefish, they get pretty big.
One thing that will really help us out is the temperature of your pond. Footprint might also help, more of the shape than the size though - since even at 4 feet depth, 18,000 gallons provides you with over 600 square feet of pond.
