In db4o locks can be implemented with the help of Semaphores. Though you cannot implement page or cluster locking, you can still vary the range of locking by using different semaphore names. For example:
.NET:
extObjectContainer.SetSemaphore(typeof(Pilot).Name,
3000)
.NET:
extObjectContainer.SetSemaphore("LOCK_"+objectContainer.Ext().GetID(pilot),
3000)