The answer of the fourth riddle - Hidden truth
Okay...
- The first line "Shined even in a little light"
'It' shined in the light?
- The second line "The true expressions were hidden..."
'It' can hide the person's expressions?
- The third line "By the wisdom and the secrets..."
You can wear 'it' to hide the secret of who wear it?
- The fourth line "...The dark truth which was disguised..."
Truth = secret /was\ disguised = hidden
- The fifth line "...And still remained..."
What was left from wearing it (To disguise/hid the secrets)
- The sixth line "...In a shape of glinting eyes..."
'Glinting eyes?' Whose eyes always glinting in the light? (because that person wearing 'it') Yep! It's Conan-kun!
So what Conan always wear to disguise or rather, uses as a disguise? and also shines in the light? Definitely it was obvious... His Glasses!
*From Gradual's review* - The answer could be Kaito/Kid's monocle too, but the words 'wisdom' and 'glinting' indicate more of Conan.