I don't even understand the question for Day 4. Well, I do a bit - I vaguely remember some of the jargon.
I'll spend some time on google - that usually cures all ignorance.
But, at first glance, it's either stupidly difficult (write code to do MD5 hashing (whatever that is)) or just boring (keep chucking numbers into existing MD5 functions until you get one back with the appropriate number of zeroes. It's possible I've misunderstood!