Firstly I want to say that those cars already shown for Round 4 all look like very nice steps forward, I'm really curious how it turns out.
As my car is generally quite new, I had a lot of areas to improve, so here is my car for Race 4:
 
Quite some changes, as I could finally concentrate on performance after my cooling was good for the last race. In detail:
Frontwing: 
heavily reshaped profiles for flow distribution and balance
Front Floor: 
repositioned outer fence, inner fences got a concept change, together with a shape adjustment of the floor leading edge and side edge. Should lead to a cleaner main floor vortex.
Sidepods/Enginecover: 
Also a complete redesign. Wider sidepods, smaller inlet, larger louvers to be able to close to outlet down. Engine cover helped with drag and generally a cleaner flow towards the rear.
Diffusor:  
Optimized volume and shape towards the beamwing, small cutout control the main vortex blowup. 
Rearwing:  
Some of the high downforce profiles I didn't put on the car before Monza, mainly a much more aggressive mainplane, flap and beamwing. Not quite happy with the overall rear of the car though.
Should be a nice step forward, but as my correlation wasn't good until now I'm not risking any prediction. Mostly concerned about balance at this point.