What to improve in VW Golf Mk4?
 

Currently I'm achieving around 4L/100km(59MPG) at 90km/h(55mph).

Is there any way to improve this with stealth aero mods?

with stealth
 

No.

I was thinking in grill blocks, underbody smoothing with flat panels, airdam,..
Isn't it effective in this kind of car?

this kind
 

Volkswagen wrote the book on aerodynamic detail optimization,beginning in the mid-1970s.It's extremely unlikely that you could improve on what they've already done,short of spending $100,000 US in a full-scale wind tunnel,doing tweaks.And none of that would really show in fuel economy.
That said,you could do some things that would really make a difference,but none of it would be stealthly.All the mods would be immediately obvious to the most casual of viewers.
Lower drag is about the 'look.'
From where you're starting from,all the low-hanging fruit is already picked.

Ok, I didn't knew it was already that optimized. I used to think that my Golf was a brick with wheels.

What you think about using using CFD analysis to try some tiny bits and see how effective they are?

I already have a model of my car that I could use for that, but need some ideas to start.

https://imagizer.imageshack.com/img921/6470/VXNWxG.png

CFD
 

All you need to do is build that blue area behind the GOLF,along with rear wheel skirts and you're done.It's pre-engineered.It's been a fact since 1922.

Thanks, so nothing I can do. I can't make it road legal here in Portugal that way.

can do
 

Are you allowed to attach a cargo rack to the back of a vehicle? If so,perhaps it could be streamlined and at least you'd get 610mm additional length.You'd see a difference at the gas pump with that for highway driving.

aerohead's trolling you. I completely support that, it's not personal. Just that the answer has dependencies. Like local regulation. And what is exactly stealth.

Who defines stealth? As they say you can not see what you do not know.

Stance and plasma actuators* might do a lot. VW engineered to a general use case. Your example might be re-optimized for a more specific case.

Countershaded camouflage*?
Nothing worth doing is easy. Boxed cavity*?
This is of interest to me, but I'm just following developments. Those are VDB* and Blender 2.8*.

*Details on request.

Mk4 like the 1.8L turbo motor?