you can see 4 first line here. 3 nodes off-line in waiting list (not validating at current epoch) and another one eligible (validating at current epoch) , perhaps currently syncing and will be seen online in few minutes.
https://explorer.elrond.com/nodes?type=validator&online=false
no some nodes are in waiting list (not eligible in current Epoch) and they can be in maintenance.
This stuff ain't my speciality, best of asking in validators group mate.
^just becacuse 4 validator nodes aren't ACTIVE doesn't mean there are 4 free slots
you can see 4 first line here. 3 nodes off-line in waiting list (not validating at current epoch) and another one eligible (validating at current epoch) , perhaps currently syncing and will be seen online in few minutes. https://explorer.elrond.com/nodes?type=validator&online=false