The four frames of reference can be reduced to two: physical and mental phenomena, or - another way of putting it - body and mind. Even though they are divided into four, it is simply the current of the mind that is divided. When you come to the essence of the practice, it all boils down to the body and mind. If you want really to simplify the practice, you should focus on investigating the body and then focus on investigating the mind.
1. To focus on investigating the body: Be aware of any one aspect of the body, such as the breath, and then when you can keep focused on it accurately, spread your awareness to observe other aspects of the body, examining them from various angles. While making your investigation, though, don't let go of your original focus - the breath. Keep examining things until you've gained clear and true insight into the aspects of the body, and the mind becomes more quiet, still and subtle than before. If anything arises while you are investigating, don't fasten onto it in any way.
2. To focus on investigating the mind: Set your awareness at one spot or another, and keep that awareness perfectly still. After your mind has been still long enough, examine the ways it then changes and moves, until you can see that its movements, whether good or bad, are simply a form of fabrication (sankhara). Don't let yourself become preoccupied with anything you may come to know, think, or see while examining. Keep your awareness in the present. When you can do this, your mind is headed toward peace and clear insight.
It's definitely not like riding a bicycle, if you don't practice for a while it's like starting from scratch!