PC partitions are a bit limited and awkward for historical reasons. You can only have 4 primary partitions,
sda4. The reason is that the primary partition table in the boot sector has only room for 4 entries. If you want to have more partitions, you need them to be logical partitions. Logical partitions are contained in an extended partition, so to have a logical partition, you must have an extended partition. That extended partition requires an entry in the boot sector, of which all 4 are already taken.
Delete one of the primary partitions, create an extended partition, and make as many logical partitions as you want. (Well, up to 11 of these, as Linux only supports partitions numbered up to 15.)
Linux doesn't care whether it's using primary partitions or logical partitions.
Since the disk is presumably going to be used only under Linux, my recommendation is to forget about PC partitions. The point of them is that they are understood by every operating system. If you aren't sharing the disk between multiple operating systems, they're irrelevant. Use Linux's native partitioning system instead: LVM. Make the whole disk an LVM physical volume, make a volume group containing just that physical volume, and create as many logical volumes inside that volume group as you like.