Assuming you haven't already created a public/private key on the Controller Pi run the following.
ssh-keygen -t rsa
Press enter to save the key at the standard location (/home/pi/.ssh/id_rsa), you can optionally enter a passphrase but you will need to enter this every time you access the key.
Copy your public SSH key to the Pi Zeros
for I in 1 2 3 4; do echo "Copying to p$I.local";ssh-copy-id pi@p$I.local;done
For each Pi you will need to type "yes" to connect if you haven't already previously logged into the Zero from the Controller using SSH and then enter your password.
You will now be able to log into the Pi Zeros using SSH without any key
To run a command on all Pi Zeros sequentially you can reuse a portion of the above command.
for I in 1 2 3 4; do echo -n "p$I:";ssh pi@p$I.local uptime;done
You should see something similar to the output below which shows how long each Pi Zero has been booted, number of users, etc.
p1: 21:43:32 up 24 min, 3 users, load average: 0.01, 0.02, 0.00 p2: 21:43:33 up 6 min, 2 users, load average: 0.07, 0.08, 0.05 p3: 21:43:33 up 5 min, 2 users, load average: 0.00, 0.07, 0.04 p4: 21:43:34 up 4 min, 2 users, load average: 0.02, 0.11, 0.06Tags: cluster hat, clusterhat, keyless, login, SSH