I beg to differ: ext4 is an excellent choice for USB media. What makes you think it's not?
As for "Linux only", that depends on the level of expertise of the people you want to protect the content against. There is software to make Linux filesystems accessible on Windows, mostly on the level of file managers that have support for them built in. Just using a filesystem type that doesn't work on Windows by default is only a very low-level protection against amateurs.
If you really want your USB stick to be secure against unauthorized access, why not encrypt it? That requires some one-time work to set up the encryption, but once that's done, using it is quite simple.
See How to encrypt external devices?
Please notice that you will of course have to set up suitable permissions for directories on that USB stick so you your normal user account can actually use it:
cd /media/$USER/mystick # or whatever the volume is named
sudo mkdir mystuff
sudo chown $USER mystuff
See also