The maximum work space for persistence is 8 GiB (not 4 GiB). The casper-rw file is maximum 4 GiB and the home-rw file is also maximum 4 GB. But there are restrictions, max 4 GiB for the system (installed programs) and max 4 GiB for tweaks and personal files.
It is possible to have an extra data partition for huge files like video clips, so maybe these 8 GiB will be enough for many users, who want multibooting with persistence for more than one of the systems.
This works with a FAT32 file system and with adding
persistent-path=/(folderx)/
after "persistent" in the grub2 menuentry for each iso file to get separate storage for persistence in a multiboot pendrive.
Boot the MultiBootUSB drive.
Open Nautilus as root, go to computer/cdrom and make a folder named Shared Data.
Save any stuff you want to share with a Windows computer there, You will need to be root to access the folder.
When plugged into a computer running Windows, the Shared Data folder will be accessible when the drive is opened in Explorer.
Windows can only see the first partition on a flash drive and MultiBootUSB needs to be installed on the first partition, but they can share it.
Mkusb will create a Ubuntu flash drive that boots using grub2.
It uses a FAT32 boot partition, a ISO9660 read only OS partition, an ext casper-rw persistence partition, (not limited to 4GB), and a NTFS partition that can be used by Linux or Windows.
https://help.ubuntu.com/community/mkusb
You can split the casper-rw persistence partition into casper-rw plus home-rw if you want more persistence options.
It is a little work to turn a mkusb drive a multibooter but it can be done by editing grub.cfg to boot iso's located in the usbdata partition.
Up to 14.04 it was possible to add a persistent partition to syslinux type Live drives such as Startup Disk Creator and Unetbootin make, now days this just results in BusyBox.
Best Answer
Ubuntu 19.04 and earlier
MultiBootUSB is a cross platform software written in Python which allows you to install multiple live Linux distros on a USB disk non-destructively and has an option to uninstall distros. From the MultiBootUSB Guide: