Okay, so I am the guy who uses a lot of linux and is using Ubuntu 12.04 right now. I have had quite a lot experiences with USB booting, so I can assure you that 8GB would not be just enough.
You should at least get a 32GB USB Stick for using 4 OS in the same stick.
Also if you want to use Multi Boot USB stick then it would not be easy for someone like you who does not have experience with linux that much.
I would recommend you to use your older drive and create Single OS Sticks first and gradually learn and learn things. Multi boot USB is not something that you should target from day 1. You would have to create Grub Menu, Partitions and lot of other stuff which I am not sure that would you be able to make or now on your own.
So my recommendation would be to go for a single OS on a single stick first and gradually learn from it.
To make Live USB, the easiest software is Unetbootin. It is easy and straightforward to use. Just open the program --> Select the .iso file --> Select Persistent storage (For Ubuntu based OS) --> Next, Next, Next and you are done..
You can get Unetbootin from http://unetbootin.sourceforge.net/
So, first get experiences with single OS and then try to jump into using Multiple OS from the same drive.
I hope this helps. Is there anything else I can help you in?