NCR1860A question. ( Opus BT-C3100 Charger)

I am a super Li-Ion noob.
I have (4) protected NCR18650As. I put them on the charger last night, but removed them before fully charged (around 4.0v) when I went to bed. This morning I put them back on the charger and when I went to check on them, it was in “discharge mode” … I then switched it to charge when the cells were around 3.5v.
My question/concern is, can doing this damage the batteries? Should I have left it alone and let them discharge fully?

No need to discharge li-ions fully. No harm either way.
Discharging below 3v can do damage tho.

Thanks for the response Helios!
Part II
I admit I didn’t search :~
What is the best voltage to charge at? Default is 500 mA.

depends how soon you need them.
slower is better.

HKJ suggests 1A charge if it has that mode.