Thanks again @dylanw, that did the trick!
For future reference, here are all the commands I used:
fdisk /dev/sda
p
n
<enter> x 3
p
w
pvcreate /dev/sda4
vgextend pve /dev/sda4
lvcreate -L 30G --name iso-storage pve
mkfs -t ext4 /dev/pve/iso-storage
mkdir /mnt/iso
mount...