The selection rate for LCDR is around 80-85%. The selection rate for the "control grades"--which are O-4, O-5, and O-6 is governed by the Defense Officer Procurement and Management Act, commonly known as DOPMA. This law sets the selection rate percentage range and flow point (years of service) for the control grades. Services sometimes have to operate outside of the designated ranges, and when they do so it typically requires the service secretary to grant a waiver. If you're thinking of doing an interservice transfer, you may not want to wait too long. Each community expects junior officers to complete various experience tours. It might be hard for the Navy officers on your O-4 selection board to judge your Army assignments against what is typically expected for a Navy AMDO. That is something you should consider as you pick your timing.