Can you post a link to these online tests? Would be interesting to try.
But bear in mind - 4/6 is not much more significant than random guessing.
They are many of them, I don’t remember now the link of what I used more times (the 6 tracks one, it has one of Mozart piano concertos by Murray Perahia which was the easiest one for me). It was posted by one of the members in a discussion thread about hi-res if I remember well.
In this case, the test offers 3 different options, only one is looseless. So probability of 4/6 is combinatory number (6,4) times 1/3 ^ 4 times 2/3 ^ 2 which gives 15 * 1/81 * 1/9 and this is 2 % roughly.
Is true than one of the offered qualities is 128 kbps, much easier to distinguish from CD loseless.
I will try to find it again, I was searching them for my girlfriend who wanted if she can distinguish also.
But as I tell before, once made a couple of times I can guess all the tracks, so is clear for me that is audible. It was clear before also, in acoustic piano tracks the mp3 or vbr files give some messy low mids and bizarre rounded mid highs (lack of harmonics I think) and is quite noticeable. Again I can live without it, but now with Tidal subscription 11€/month the choice is easy for me!
To the matter of hi-res a did another testing, even if was controlled by my girlfriend and non matched levels, I cannot differentiate CD for 24 / 96 or 24 / 192 at all, nothing, zero, niente