Could not mount disk4s1 after erase?

if its blank, ERASE it again. if you interchange this card with other systems, choose GUID partition Map and ExFAT format (required for Windows Volumes over 32GB total size.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

File system formats available in Disk Utility on Mac - Apple Support

.

The gray text under the item-name, 'Apple SDXC Reader media' says that what you are looking at is a

'Secure Digital Internal Physical Disk -- GUID Partition Map'

you can't mount an internal physical DEVICE. You can only Mount VOLUMES.

You have not pictured the left column, which may show a Container-disk and a Volume-name, indented under under the physical disk.

Usually after formatting, the Volume is mounted automatically.

after the erase step. what error messages are shown, exactly?