The only restriction here is budget limitations.
Infinite answers good answers, most end with Shapton.
For a cheap upgrade, and to see where your current set-up stands, getting a good 8,000 grit stone to follow that 4,000 would be a great idea. If the 8,000 gets your knives sharper, then you know your current setup is decent enough. If the 8,000 doesn't have much effect, then you will have to upgrade the 1,000 or 4,000 stone. Having a good course stone and diamond plate are essential in my opinion.
Let's let someone else chime in