That's how arbitration works, the team initially lowballs the offer. It isn't indicative of how much they really think he's worth.
This.
It's not like MLB arbitration where the award is an either/or decision between the figures submitted by both sides. That restriction means that a baseball team has to provide a stronger, more representative salary figure as their offer in order to have it get serious consideration.
NHL arbitration allows the arbitrator to pick any amount, generally from in between the requested figures of both parties (though I don't believe this is set in stone as necessary), so it makes as much sense for the Jets to come in low at $4m as it would for Trouba's camp to come in equally as high (say, $8-9m) and then the arbitrator can find a middle ground.
It almost makes the NHL setup more like arbitrated/mediated negotiation instead of MLB's mark-your-line-in-the-sand binding decision of whose offer is more "right"
It's even part of the reason you see so few NHL cases actually ultimately rely on the arb process to make the final decision instead of figuring out a reasonable settlement before or just after the hearing.