You can do that in Ubuntu without formatting them as ext4, though I recommend using Ubuntu Server since you can install Samba during installation.
With Samba, you define share and set options, like authentication and permissions, and share your defined shares on your LAN.
But I will go over a generic install, first we will need to install Samba, then edit the config file, then restart the Samba services and we should be good!
Let it finish installing and then we will need to edit the config file, so type:
sudo apt-get install samba
Scroll all the way to the bottom and type:
sudo nano /etc/samba/smb.conf
Don't type the text in the parenthesis.
Press "ctrl+x", and possibly type "yes" if it ask, to save the file.
comment= My New Share
path= /path/to/directory (Can be any path, ex. /home/chris/share)
guest ok= yes
read only= no
That will make an open share that anyone can access and put/take anything from it. We can lock it down and make it more secure if you want, just let me know and I will show you how to do that.
Now we need to restart the samba service, type:
That should get you up and running with a Samba defined share that anyone can access when they connect to your LAN, let me know if you have any troubles and I will help!
sudo service smbd restart
sudo service nmbd restart