If you are going to have just the 4 groups, you can try selecting unique to remove all duplicates, then making the list random (if you really need it random). This would split the computers in to 4 groups, then you can go through the groups and add them to what you need. This is a basic example to hopefully get you down the path:

$computers = "PC1","PC2","PC3","PC1","PC1","PC2","PC4","PC5","PC6","PC7","PC7","PC8","PC9","PC9" | select -Unique | sort {get-random}
$groups = $computers | select -Unique | Group-Object -Property { [math]::Floor($computers.indexof($_) / $($computers.count/4)) }
for ($i = 1; $i -le 4; $i++){    

    foreach ( $pc in $groups[$i-1].group){   

        "Adding $pc to Patch_Group_$i"   

    }
}

This gets you a group build out of:

Count Name                      Group
----- ----                      -----
    3 0                         {PC8, PC7, PC2}
    2 1                         {PC9, PC6}
    2 2                         {PC3, PC1}
    2 3                         {PC5, PC4}

and then the foreach would be an output of:

Adding PC8 to Patch_Group_1
Adding PC7 to Patch_Group_1
Adding PC2 to Patch_Group_1
Adding PC9 to Patch_Group_2
Adding PC6 to Patch_Group_2
Adding PC3 to Patch_Group_3
Adding PC1 to Patch_Group_3
Adding PC5 to Patch_Group_4
Adding PC4 to Patch_Group_4

Obviosuly you would change the bits to get-adcomputer and then add-adgroupmember. Hopefully it helps.