I've not heard of the 4 finger rule. I can probably explain to you the 3 finger rule though or "Chicken Tikka Masala"
C- CAS
T-TAS
M- MACH
Lets try an example.Using your thumb and next two fingers on your left hand with your palm facing towards you. Your thumb is CAS, middle finger TAS and other MACH.
Now, imagine you were to climb at a constant CAS. What you want to figure out is what is happening to the TAS and the MACH. So CAS is a bench mark for this example and therefore we need to do something with our thumb.
Rotate your hand so that your thumb is pointing straight up in the air. Whats happened to your other fingers? They're both pointing to the right, right? Or if you imagine that is a graph format, they are increasing. So TAS and MACH increase
Look at it the other way, if you were decending at the CAS, so starting at the top of your thumb and coming vertically downwards, so coming back down the fingers they are now decreasing also. So decending at a CONSTANT CAS the TAS and the MACH decrease.
Now rotate your hand so that the middle finger is pointing upwards.
So a climb at constant TAS, the CAS decreases and the MACH increases.
I'm not the greatest at explaining things but hope this has helped you.