The fish you desribe sounds like it is definately a juvinile TSN,a absolute aquatic monster that can and will consume fish up to two thirds of its own size,they are not fighters mind you and will be picked on by fish such as cichlids,large barbs and charicins,that is until night time when the catfish will just eat them while they sleap,trust me another member just found out the hard way and lost a much treasured malawi cichlid to a blind TSN he had taken pity on.This fish can grow to 4 feet long,as im sure you know,and due to the huge ammounts of waste created after a feeding should only be kept in tanks of 150 gallons or more.You can not and will not tame a TSN they are out and out piscavors and view all tankmates as food,even your ray will not be safe.
I dont know where you did your research on the black arowana Osteoglossum ferreirai but id certainly like to know,this fish will not get as big as its cousin the silver O.bicirrhosum but will still attain a size of 40" which im sure you agree is too big for a 48" tank,as for how fast they grow depends on what they are fed but you can be sure this fish will outgrow a 4 foot tank within a year to 18 months.
To answer your question,no your tank is not over stocked at the moment but in the off chance that the TSN dosent wipe out the tank you will be heading for problems in the future,a 4 foot tank is only suitable for holding 4 large cichlids and the TSN and BA both get far to large,the clowns and ray will be fine.
Good luck and if possible post some pics,id love to see the tank before all hell breaks loose,the fish you have are certainly a collection of stunners.