It will last you until at least 6". Sometimes at 6" in a 12" wide tank they get really nervous and smash their faces into the glass. Since it's a 4 ft tank though, that might buy you some more time/size before an upgrade is needed.
As for the current, play around with it. My raph never really liked the current that much. Once I added a powerhead to it's tank, it stopped being active. Definitely watch the fish's reaction to high current vs average current to decide what your's will like.
no, dont think its adeqaute size. these fish are extremely skittish and would need at least a 2' wide footprint. even at juvenile size they are claustrophobic and will bash themselves imo.
I have one about 4-5" in a 55 temporarily although I don't recommend it. It seemed to calm down quite a bit after the first few days actually. When I first put him in there he was constantly rubbing on the sides to the point I thought he was going to damage his nose. Luckily not much damage occurred and he pretty much hangs out in the current until it's time to eat.
Look for posts by Vamptrev (I think) who had a fairly large one and had to keep it in a pond. They do not do well in small tanks and unless you have a REALLY big tank coming, I would not bother. They have a habit ot rubbing their eyes against the glass and will knock themselves against the sides to the point they will kill themselves. Having said all of that, if you can house one, their feeding habits are incredible.