An understandably frustrating occurrence having watched through the replay. I think the previous explanations regarding the blocked LOS from the gunner's position in the turret explains the inability to open fire. If nothing else it demonstrates the level of reality built into the game in differentiating between individual crew members abilities to spot. No averaging from the centre of the vehicle going on here.
I suppose for me it raises a couple of minor questions the first being about the efficacy of the LOS tool in the game. The information provided in-game by checking the LOS whether it be clear, blocked or partial is all a player has to go on when plotting moves. It needs to be a accurate representation of what is "likely" to happen. The replay shows a red target line and presumably, although not shown, the LOS showed as unblocked in the preceding plotting phase. From memory grey target lines from vehicles tend to arise when weapons are blocked i.e. hull down with text to clarify. Do vehicles generate grey partial spotting lines in the same way that infantry do where some soldiers cannot see the target? I would hazard a guess not although I stand to be corrected on that. In this example during the plotting phase W.2010 will have taken on face value the information fed to him by the game - target visible and LOS clear - engage. A grey partial spot " gunner LOS blocked" would have triggered a different response for him, perhaps a "hunt" move forwards to acquire a firm targeting line or a reverse backwards to what appears higher up the slope so as to clear the obstacle in front of the Mk. IV. There would have been no need to query the forum to clarify what was going on.
Secondly, the Pz IV seems to make no attempt to rotate the turret to match the targeting line. That raises a question in my mind on how the "in-vehicle" AI routine processes this situation. The historical reality would pan out along these lines: TC - "Target - 1 o'clock - 600 metres" (or the German equivalent). Gunner rotates turret to bearing - "Sights blocked - cannot see target". TC responds with fresh instructions. That doesn't appear to happen in this situation which implies the AI "knew" almost immediately from the calculations that the gunner could not see and therefore did not bother to rotate onto target to no purpose.
From a programming point of view I would imagine it is considerably easier to incorporate a gunner LOS check at the point it is needed i.e. the moment the vehicle is told to engage rather than create an "in-vehicle" AI subroutine sufficiently complex to handle all conceivable situations as they arose. It is a somewhat circular argument in my mind as either approach would deliver a similar result - no shot in that circumstance was possible. The solution, it strikes me (if it does not already exist), would be a more precise LOS tool differentiating whether all relevant crew members of a vehicle can see a target.
I hasten to add that I have not had an opportunity to search to see if this particular point has been discussed elsewhere and whether information and a consensus already posted up. I would be particularly grateful if any kind soul could clarify or add to this subject.