Reverse engineering a chord sequence?

Hi, Lee. Cracking website – thanks for making the effort and putting it out there.

My question: Having put together a four-chord sequence, are there any rules that dictate where the sequence might go from there? Should I write a melody line that passes chord four and gives me a clue?

I can find a chord five that fits but the sixth is another matter… 🙁

Cheers, Gary.