I'm pretty sure that it needs to maintain contact/connection with the ST16 as that is how the dynamic RTH feature is set to work. I guess, if it lost connection at some point, and the ST16 hadn't been moved, then it would (should) still return back to that point.
I gather you are doing this work for the University? Maybe their budget is set up in such a way so that you can trial this and if you lost the H, then that may be factored into the budget?
I would also be wary of the battery life. You say this has been calculated, but very often, what looks fine on paper (or computer) doesn't necessarily cross over to the real world. You have outside factors to consider like the wind, speed of the H, etc. Also a 4km distance from point A to point B usually turns out to be a lot more, particularly if you are not just going straight out and back. If you don't think you can get this total flight done (from take-off to landing) in less than 12-15 minutes then I wouldn't be risking it. Not to say you can't stretch the time out more, but you are going to start to move into territory where you are going to place your battery at risk.