Fourth finger lies too low so very often 'plays' along with another correct note being played. What might be a good solution?
Drills to improve the dexterity of individual fingers are often called 'finger independence exercises'. One that helped me immensely went like this:
Place one hand on the keyboard in a proper playing position. Take one finger and hold down the key underneath it, then go up and down with the other fingers similar to playing a scale.
For example, while holding down a C with your right hand thumb, play D (index), E (middle), F(ring), G (pinkie), and then back down. Play very slowly and concentrate on holding the thumb in place while moving only the intended finger. Then do the same drill with each finger as the 'anchor' finger that doesn't move. The most important thing is not to rush, and to carefully press each key with only the finger you want to move while keeping the anchor finger firmly in place. More complex exercises can be included as you master the basic coordination.
Also, when you are doing routine scale practice, focus on the movement of your ring and pinkie fingers, which are the most likely to move together. If you hit both the ring and pinkie notes at the same time, stop, reset, and do the movement correctly. Speed and accuracy will naturally improve with time and effort. The most important piece of practice advice I ever received was that it doesn't matter how many times you do it, it matters how many times you do it right.
Alex Y suggests an excellent exercise but I think it’s quite difficult. Of course we don’t know how you play, but moving two fingers instead of one suggests you are not very advanced yet and I am concerned that you would only be able to play Alex’ exercise with a lot of tension in your hand which is never good.
An alternative exercise that is easier in my experience (yet still very useful) is as follows:
You need a melody of five adjacent notes; CDEFGFEDC works fine but might become boring after a while and this exercise works just as well with (for example) CEDFEGEFDEC so feel free to mix it up (but I do recommend to only use 5 adjacent white keys and to use all fingers approximately the same amount of times). Then play your melody by lifting up each finger before you press it down. Take special care that you use only the finger muscles: you shouldn’t need to flex any muscles in your hand or wrist. (At first you will not be able to relax completely but keep trying!) After some experimenting I realised it is much better to play the melody legato, instead of separated as I suggested in an earlier version of this answer.
So in summary:
- Lift finger above normal resting position (with only finger muscles)
- Press the key
- Play the next note the same way, releasing the previous key as you strike the next one.
Exercises should definitely help solve your problem. I would recommend having a number of them that you do every single day. Practice them slowly and evenly with a metronome on a slow tempo. When you have the exercise perfectly under control, increase the tempo by one notch. Here is one basic pattern I have used for years, and I would highly recommend it:
Position your right hand thumb on middle C, your index finger on E, middle finger on F, etc. Only leave a space on D between your thumb and index finger. Play the notes in order, ascending and descending until your index finger plays E. Then immediately shift your thumb to D. As you play D, shift all your other fingers up one key, leaving a space (still between your index finger and thumb) on the key E. Play F with your index finger, G with your middle finger and so on. There should be no pauses between any of the notes. Concentrate on releasing the previous finger immediately as you play the following. Move up the keyboard in following pattern: CEFGAGFEDFGABAGFEGA etc. until your thumb is on C one octave above. When you have reached C, hold it, and then start to move the other way, leaving a space between your pinkie and ring finger, starting on G like this: GEDCBCDEFD etc. Always follow the fingers 123454321 ascending and 543212345 descending.
Do the same exercise with the left hand, only in reverse. Ascending, start with your pinkie on C and leave the space between your pinkie and ring finger. Descending, start with your thumb on G and leave the space between your thumb and index finger.
It is a common misconception that we play the piano from the fingers. We don't. We play from the arm which places the fingers. When we watch virtuoso pianists play effortlessly and it appears that their fingers are relaxed and not moving it is because they are not playing, the work is being done by the arm. Thus, teachers try to teach their students to play with a relaxed, still and quiet hand but never tell them what to use instead. It is the fake news of piano pedagogy. If a student has cramps, the teacher will instruct them to relax but not tell them what to use so they use the same muscles they are trying to relax.
It is true that all five fingers must play in the same direction at the same time and all five fingers "play" at the same time but only the ones you want to actually depress a key will have the slightest flexation to play down on a key. The rest just sort of "tap" the keys.
If you are a finger isolationist, there is nothing anyone can tell you to fix your problem. For sure, more isolation exercises will not fix it. Never isolate or lift A finger high. Drinking more alcohol will not make you less drunk. It would be like trying to tell someone who walks barefoot on broken glass every day how to stop their feet from bleeding. If they're not going to detour or wear shoes, there is nothing to say.
My suggestion if you play from your long flexors is to find a new teacher and overhaul your entire technique.
Although, my short answer is that you probably lack proper alignment of the forearm, may have an ulnar deviation of the wrist, or have no shape to your playing apparatus. That can all be fixed by getting your playing in your elbow and shoulder.
Remember from your HS physics class and Newton's law of physics: Every motion has an equal and opposite motion. Since the piano is down, before you can play down you must first play up. Once you get "up" mastered you can minimize it and nobody will be able to tell that gravity is doing all the work. Observers will think that your hand is relaxed and really, it is because your arm is doing the work.
Heed my warning now. One gaze into my crystal ball reveals an "itis" or dystonia in your future. The good news is that same aforementioned teacher can fix those, too. Find her now. On ounce of prevention . . .
Most piano-like teachers need to be swiped left.