I don't disagree with you
From the Werknummer range you are looking at you are trying to tell whether this was a late G-2 or early G-4 from Wiener Neustadt. I think that the features you are discussing (cockpit ventilation, upper wing bulges for the larger 660/150 main wheels and the alteration in undercarriage geometry) are not so definitive between such closely related aircraft. We know that late G-2s got the bigger wheels and some early G-4s got the smaller ones for example. I've never been sure about the ventilation inlets. They disappeared fairly early in G-2 production as far as I can tell.
The G-4 was supposed to have the FuG 16Z radio installation and the different position of the antenna lead into the fuselage might be your best clue, if you can find an image on which it is visible.
If I had to bet, I'd put my money on the aircraft in your picture being a G-2, but I'm not a betting man !
I will try to have a look for images of Sachsenberg's Bf 109s when I have a chance.
Cheers
Steve