I tried 2 different EM4100 readers, and none worked. I have looked into the encoding, unless you find a “universal” or “multi-standard” RFID reader, it won’t read the HID cards. Same in the other direction, an HID reader will not read an EM4100 card.
I have 100 card readers and 600 cards, I am slowly developping a Raspberry Pi based solution based on the HID readers, no choice. If you have only and handful you can do a flag-day swap, but I can’t.
The frequency is the same, and in theory there is such thing that could read both formats (which is why I gave it a shot) but in practice I have not seen anything out there that works.