If you really want to use 4 disks for a RAID1 array, I suggest you to go with a 4-way RAID1 array. In this manner, should a disk fail, you continue to have triple-protection (3-way array) without needing any rebuild.
If you really want a 3-way mirror + hot spare, you can use mdadm --manage --add-spare
to add a spare to the RAID1 array. For example, lets start from this 3-way array (note: I am using loopback devices, while you want to use real disks):
[root@localhost test]# cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 loop2[2] loop1[1] loop0[0]
7168 blocks super 1.2 [3/3] [UUU]
Adding a spare is quite simple:
[root@localhost test]# mdadm --manage /dev/md127 --add-spare /dev/loop3
mdadm: added /dev/loop3
Lets check /proc/mdstat
now:
[root@localhost test]# cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 loop3[3](S) loop2[2] loop1[1] loop0[0]
7168 blocks super 1.2 [3/3] [UUU]
Again, be sure to use the correct device name (rather than blindly copy/pasting the above commands). And take a confirmed-working backup first.
However, I consider a 4-way or 3-way+hotspare RAID1 array overkill. While for extremely important data a 3-way RAID1 mirror can be justified, you should really plan for failure scenarios in which the entire server is down. In other words, don't skip on regular backups simply because you have mirrored disks.