I have a 4Tb. drive that is currently partitioned into 2 x 2Tb.
One drive contains my ‘home’ directory and the second drive contains only my ‘pictures’ directory.
It now appears that the pictures director is to small and the home directory is too large so I wish to change the partitions size and then reinstall the entire system.
gparted is an intuitive graphical one, cfdisk would be a kind of menu driven command line tool.There are more.
When you repartition, you’ll lose the data on that drive - have a backup of everything on a different drive(!),
repartition and then copy it back from there.
gparted could be used to resize partitions with the data on them and preserved, but something could still go wrong
and it will take a long time to shuffle the data around as well.
Don’t do that without having a backup copy of everything.
I only have experience with spinning disks - it’s probably not as significant, time wise, with ssd/nvme.
The choice of file system also plays a role.
With BTRFS for instance it is easier to adjust the size of “partitions”, even on the fly, I think,
compared to ext4, for example.
Then there is LVM, with the file system of your choice inside it.
Vaguely similar to btrfs.
I am used to using gparted but do not find anyway to re-partition to a single partition. The software insists that 2Tb. is the maximum allowed.
Am I missing something?
MBR/MS-DOS is limited to 2TB per partition. For more than that, you have to use GPT. This means a complete wipe to change the partition table when switching it.
This might also be the right time for a clean install, as it’s better for the future to use
Of course, all data from /home can be retained. 
P.S. and don´t forget a swap partition 
Heck, it’s a long time since I went for resizing partitions… and it seems like half the time I wish I’d never started… best avoided if it’s possible and maybe 35% chance that you will wish you had a backup if you don’t have one.