If one wants to develop Karuna, then suppose that when you are sitting there is physical pain, there is suffering in yourself. So rather than continue to suffer as a result of that physical or mental pain, you try to do something about it, you learn to be friendly in relation to it, you learn to let go of it. This would be practising Karuna in relation to meditation.
And when you do that you can immediately experience sympathetic joy and feel happy: very nice, joy instead of suffering; I tried to do something about it and now there is a change taking place. So you can experience joy about the pain, happiness about it. Then you can experience the last quality, realising maybe I should not hold on to this joy, I should have a non-reactive mind to the joy.
So you see how in one particular situation, a simple situation, one can develop Metta, Karuna, Mudita, and Uppekha, all four qualities.