Yes, iBank for iPhone will sync with iBank 4. However, certain features will not be available when syncing with iBank 4. For example, budgets will not sync between iBank 4 and iBank for iPhone. Also, Direct Access will not be available when syncing with iBank 4.
Since iBank 4 is no longer being developed, any future enhancements and improvements made to iBank for iPhone will only be available with iBank 5.