The other 3 Founders need to speak with the problem founder. Depending on your dynamic, this can either be as a group or a simple one-on-one with a designated Founder to speak for you three. The one thing you want to avoid is having this look like a plot to band against Founder C. It makes it easier if they don't become defensive immediately.

In this meeting, mention exactly what you have here:

 - Low productivity
 - Mood swings that affect the team
 - Unreliability

Listen to their explanations and keep the conversation on track and civil. There may be valid, temporary reasons for all of these (school, vacations, sick family members, etc). If they are temporary, productivity may improve soon.

Next, ask what can be done to improve the reliability of Founder C. Two of your three complaints are related to productivity directly, and mood swings can impact it as well. Let him offer suggestions, but be prepared with your own. You do have some options:

 - Step down as a founder to one of the lower levels. Presumably these are less involved and stressful and don't require as large of a time commitment.
 - Step aside completely but remain as a core community member
 - Put in X hours per Y days in an effort to prepare the server(s) for new community members

Set a time line for when improvements are expected. Make it known that the other Founders are committed to launching the community and his involvement is important. If real life is interfering with that commitment, he needs to realize it and either step aside or find a way to balance the community and life. If he can't, and performance is important to you and your co-founders, he may need to be removed.

It's important to remember that you can't force them to do things. This isn't an employee and you aren't their boss. From your description, you guys are on equal footing. If you do end up removing this person, be prepared for a bruised ego. However, by having discussions with them and setting expectations on when problems will be resolved, you may be able to prevent a lot of bad feelings from cropping up.