EXT3 is the way to go if you are looking for compatibility. Puppy will be able to read ext4, but you will first have to manually load the module that allows the kernel to recognize the filesystem. The thing is if things go wrong, you don't want to waste time struggling with that as well. But by all means, as the blokes recommended. Sticking with EXT3 is a good choice, I have seen very few distros that does not support it natively.
If you have no important data, reinstall with EXT3, if you want to play around with the linux system and see how things are done, you can read here http://www.troubleshooters.com/linux/ext2toext3.htm
on how to convert the filesystems from ext2 to ext3.