The key to these four levels is to start fishing from the surface of the water to the bottom, this allows you to cover various depths to see where fish are holding. It also allows you to fish various baits in all of these depths to see what the fish are interested in.
This will increase the amount of fish you catch while you are fishing and because of the reactions to this process you will quickly find out what the fish are going after. Key thing is when you find what is working, stick with it until it stops working, don’t just catch a fish or two and try something else, stick with that bait and depth presentation and “exhaust” it until the fish stop biting.
When the bite stops, then you immediately try another bait in that same depth, yes the depth will more then likley still be the same but the style, size and colour of the bait will have changed.
This process should be repeated at every depth, you will quickly see that your fish caught ratio will go through the roof very quickly, I “never” stop casting a bait especially if it is still working, catching more fish and learning how you are catching them is very valuable, much more valuable then yelling out loud that there are no fish in this lake and that the bait you spent $8.95 catches nothing !