How are two people related who have a common 4th great grandfather? I've tried to calculate it, but have only confused myself.

You work it out like this:

If the *closest* common ancestor of both people is their.....

grandfather, the relationship is 1st cousins
great grandfather, 2nd cousins
2x great grandfather, 3rd cousins
3x great grandfather, 4th cousins
4x great grandfather, 5th cousins
etc.

Hope that helps!

Christine