I do not understand the problem you encounter. When you group speakers the one slider you see in the app controls the volume of all speakers. Only after you tap this one slider the volume controls for the separate speakers appear.

There’s just one caveat: the grouped volume control use as a starting point the separate setting all speakers are set to before grouping. If one of the speakers is set very loud, the group volume will make it even louder. Same goes for speakers set to a low volume. If you don’t want this you have to set the separate speaker volume once, when you start the group. After that first time the group volume controls should control everything okay.

Hey,

Look at my pictures, I grouped those 4 speakers together into one group but still, when I want to change the volume, I have 4 sliders for each seperate speaker instead of one slider for all 4 speakers.
I want to change the volume of those 4 speakers in the showroom with just 1 slider bar.

The reason you cannot do so is that the four are also grouped with other speakers.  There is no way (that I am aware of) of controlling a subset of grouped speakers with a single slider.

‘Room’ in Sonos is just a label for an individual speaker or particular combinations of speakers, such as stereo pairs or HT setups.  You cannot put 4 speakers in a single Sonos ‘room’. 

There's two kinds of groups in Sonos since S2. Take a look here: https://support.sonos.com/s/article/3391?language=nl_NL (I take it your Dutch). From John B's answer I gather your trying to group speakers that are already grouped via the "shortcut” method (with which I have no expericence so your picture did not trigger anything I could work with). You could try to just group these four speakers.

Damn, that’s a real pity.
It’s annoying to get the volume level the same for 4 speakers using 4 sliders...

I did just that.
I made a group ‘Showroom’ that only includes those 4 speakers!

Screenshot: https://imgur.com/a/9tyPJ8s

I can understand that but it is a fairly rare use case.  If part of the problem is getting the precise same levels with the slider, then tapping the volume bars allows fairly precise adjustments.  You could further improve this precision by setting max volume to (say) 50%.

I do appreciate that this doesn’t help with the fact you still have to do it four times.

Which is helpful, but I think when you group it with other speakers, it is the four separate speakers that still show up in the volume control.

But as I understand it these four speakers are stille part of the larger group, which according to @John B could be a problem.

This is how it works on my system;

 On my system the only slider I see with the speaker I started my group from controls group volume. So does the slider “Groepsvolume” once I tap the the slider in Huiskamer.

Doesn't "Groepsvolume”, that I also see in your picture, work for you?