Synergy - how to increase skill up to 400%? (The Witcher 3: Wild Hunt)

Good day.
I already wrote http://forums.playground.ru/witcher_3_wild_hunt/walkthrough/gerald_voin_alhimik-913973/ that I started playing not long ago.
There are many questions, and I am gradually looking for answers.
Now I read about mutagens, their methods of preparation, improvement, etc.
On the wiki witch written postscript to the article mutagens:

SpoylerSinergiya
In addition, at the mutagens, there is another very useful feature. Let's call it "synergy".
If in one group of active skills, together with a mutagen, put any skill of the same color, the mutagen bonus will increase by 100%. And this works for each next installed skill of the corresponding color, but only within the same skill group. Thus, you can increase the bonus from the mutagen to 400% of the starting value.

I don't quite understand what I mean, how to get 400% profit?
thanks
(I just put the color in the color anyway - but the gain is still 25%, I understand that I still have a mutagen (7% of the power of the signs), but how can I get 400% later?)