Several things
You cannot mix MBR and GPT - it is impossible - like fire and water.
MBR (dos) schema only allows four(4) primary partitions and there is no space available to create a partition. My guess is that the 4th partition is defined as an MBR extended partition which is the old way of having more than 4 partitions - an extended partition can hold several smaller partitions - but should be avoided on modern systems.
In any case you need ensure Manjaro is booted from BIOS otherwise Manjaro will boot in EFI mode and you get the above problem. Disable EFI boot in the system firmware.