Hi,
when running in persistent mode, allocating only 4 MB's memory is the most likely cause of you getting the "out of memory message"!.
Start Unetbootin again but don't set up the USB thumbdrive in persistence mode, set it up as a "Live CD" instead, see here for info about Unetbootin:
IT IS VERY IMPORTANT YOU PRE-FORMAT THE USB STICK TO FAT 32! BEFORE YOU RUN UNETBOOTIN.
Let us know how you get on. 