MBR - the disk partition scheme for the last 30+ years can only support 4 primary partitions. To overcome this limitation, you need to make one of the primary partition into extended partition, then you can create theoretically unlimited logical drives out of the space allocated for the extended partition, but for DOS the limit will be the last assignable drive letter Z:
DOS with legacy BIOS will not support GPT, which allows up to 128 primary partitions. If you are adventurous, then you can research further on GRUB4DOS capabilities to boot off GPT partition and the creation of hybrid MBR/GPT disk partition scheme.