The X4 is not a good choice for a Fabia or a small car. The seat is more compact than previously but still not a compact seat. Reverso is a compact seat but due to placement of the adjusting knob for recline the seat still is not that compact.
Reverso is new so hasn't been tested externally. This kind of testing should anyway be disregarded. Which!, ADAC, Stiftung Waren, etc. procue nothing more than biased and subjective garbage which is confusing for parents. Which! especially which seem to only have people who know absolutely nothing about car seats. Rear facing seats in particular.
If you want a car seat for a small car then go with Triofix/Triofix Recline which is the most compact rear facing seat available. It will also last rear facing longer height wise and can be installed with Isofix or seat belt.
If you prefer seat belt installation then Britax Hi-Way is your best choice. It's the second most compact seat and is light weight, comfortable, has passed the Plus Test to 25 kg and fits well in pretty much any car small to large.
Reverso is brand new and will soon pass the ultra strict Swedish Plus Test. I've played with the seat on the crash rig and it's very strong. It has some nice features and is simply a nice seat with a completely new design.
The seat is basically a strong frame surrounded by EPP and EPS foam. It's approved for the new R129 standard where seat plus child can weigh 33 kg. The Reverso is ultra light weighing only 10 kg. That means the rear facing weight limit is 23 kg instead of 18 for for all other Isofix seats.
Keep in mind that your Fabia has vehicle seats sloping more than usual so most seats will be quite upright. Head of your child will fall forward when sleeping in most seats. Not dangerous but not comfortable for the child. Dualfix was mentioned above and that is one of many examples of seats which can be too upright.
Hi-Way and Triofix do have very nice angle and works well in a Fabia.