it should be fine for about 1-3 years depending on the type of arowana and growth rate. although i still reccomend starting with an AT LEAST 6 foot tank. in this way u ensure the arowana will not be stressed or stunted for a few years.
why don't you made a long common tank..
that better for arowana..
i'd built long 6-8 ft tank..
since they like to swim patrolling beneath the surface of water..
anyways...GoOD LuCK
Being that your tank is basically a 4' across triangle tank with a surface area less then 7 sq ft. I would say no arowanas for this tank. I would say at best you could have one in that tank for a year.
I guess that would do for a jardini for up to 2-3 yrs.. Just make sure you get them young, about 4"-6".
You could put a g.gourami (same size) as well with the jardini.. both are aggressive so as they could defend themselves.. or you get a birchir with jardini and put some PVC pipes..