I think a 5ft long is a bit restricting to an arowana when large. But I wouldnt complain about the 4ft depth. I would skip most of those fsh, they get very large. Then you'd have to upgrade, not such a bad thing but something to consider.
$1800.00 for a hamburger is expencive... $1800.00 for a late model mustang is a steal....everythings relitive.
anyway...You should be fine with that tank for quite a while. In fact if you do eventually need to upgrade then count yourself as one of the lucky and fortunate one's who's kept a silver alive and healthy long enough to become a problem...
Bieng a fan of RTC's I hear a lot of the same nonsence...."It's going to grow to 6 feet long in a week dude...." I just shake my head and laugh. All any of us can do is to provide the best home we can for our fish and be prepaired to upgrade if and when it becomes necessary.
The tank sounds fine...and it sounds like you have your fishes best intrests in mind.