I very recently accepted an offer to start a PhD in October.

I am currently working full time in a related field for a charity. I only need to give 4 weeks notice and plan to leave at the start of September so I can relocate for the phd.

However between now and August, when I'll be handing in my notice, I have my annual review (in June) I'm not sure it's fair to me or work to go through this pretending I'm still going to be there.

Should I tell my boss I'll be starting a PhD in October before my annual review or tell them just before I plan to hand in my notice?

I would like to stay on good terms and continue to volunteer by time with the charity after I start my PhD.

nice post

Difficult to say. I used to "over care" about hurting the feelings of my employers but I try to remind myself of the fact that employers are in a stronger position than employees and the damage for them is usually much less than damage for employee.
Giving 4 months notice might be too much but if you think to a greater extent that no harm will be done to you, then do it. If not sure, 4 weeks notice is fair enough.