When I compare the 2 armours, I personally prefer the look of the Mark III.
As one member mentioned, the grey coloured parts don't match with the rest of the colour scheme.
The new red colour also has a more matt feel and look to it compared to the previous Mark III glossy metallic red. So, in a way, Mark IV looks more plasticky.
Mark III has a more heavy chest piece armour, which covers more and leaves the only a bit of the abdomen armour seen. So, this effect creates a heavier bulk look for Mark III.
Mark IV on the other hand has a smaller and shorter chest armour. Thus more of the abdomen armour is shown, thus creating an elongated and slim look on Mark IV.
I thought the Tony Stark has the umm centre light piece which is the reactor core/power source in the middle of his chest. It powers the armour as well as keeps him alive. Judging from the position it was on Mark III, and then looking at the position of the rectangle light centre on Mark IV, looks like Tony shifted his power core higher on his chest. Which is strange.
The shoulder pad armour. On Mark III, they were circular. That again adds bulk to Iron Man. But on Mark IV, the are like half oval hollow flaps.
Just my 2 cents worth.
