Not actually tested. As was mentioned earlier, the io6's ANC is also an Amp which is not present in the io4s. Brintkiks said they have tuned them to be the same but this as not been tested outside of dali.
I'm finding the volume not as high as I would like on the io6's for example and am using Equalizer FX Pro to boost the volume combined with USB media player on my phone, highly recommended BTW, to get it to where I prefer it without clipping or distortion/muffling.
The io4s may just be quiet without the ANC/Amp of the io6's or may just be louder because they don't have the ANC in the first place and don't need the amp.
It could just be my phone as someone with a S10 has said its too loud or that may be our different preferences.
My point is, the io4s can not be directly compared to io6's as been 'the same'. I don't think anyone here has io4s here?
I'm not saying anything bad about these BTW, I absolutely love my io6's and can't praise them enough, just that they are different enough that they need to be compared against each other and you can't just think that the io4s are just io6's without ANC and 2x battery no matter how much dali have tried to tune them to be the same.
Without testing, we don't know.