Muscle group 4 is first experienced as part of the anus. However, it is not. The anus is a sphincter muscle that is squeezed at rest. The muscles with the number 4 are a muscle plateau. That we often use unconsciously for a fart that is not heard.
When the muscles are trained, one starts with muscle 1 to train. Over time, a more subtle feeling arises. It is also possible to squeeze only from the front, just behind the balls. Or from behind, close to the anus. Not yet as strong as with muscle 1 but it can be done separately. This is the point at which one can consciously contract only muscle group 2 or muscle group 3.
Also muscle group 4 can eventually be contracted without the anus contracting.
When this is achieved, and the muscles have become stronger, they also start working automatically when sperm shooting comes into view. The handbrake is then applied automatically. You can now choose to keep enjoying or to shoot sperm. The latter can also be postponed until the next day.
However, it is advisable to regularly empty the prostate well. Yet this can happen on days when there is, for example, a lot of work to do and sexual interest could distract that. With proper control over the pelvic floor muscles, the prostate can also be emptied without any orgasm being felt. The sperm then simply runs out.
It is the combination of weak pelvic floor muscles, decreased testosterone production with decreased sexual interest, plus not emptying the prostate often enough that can lead to prostate problems later in life. The widely circulated notion that the penis only serves to urinate and reproduce does not help. A healthy penis should be used for what it is intended for.