Reformatting an external drive to factory - can't delete partitions

I have an external 4TB GDrive that I am trying to reformat to use for Time Machine. It had been partitioned into two 2TB drives a while back, but I don't need that anymore. It is mounting on Disk Utility as two individual drives, and I would like to merge them back to one 4TB drive. Any idea how I do that? I don't need to save any data on it.

*The names (TM and TM2) were made as I erased them. They aren't actually linked to TM as of yet.