I'm no sports medicine expert, but it is a shame no one else has answered.
Most likely you strained or slightly tore some muscle fibres of the gastrocnemius or more likely the soleus muscle. If the pain is central and you can feel a soft lump, you could have a Baker's cyst, which is a swelling of the knee joint itself that has bulged posteriorly.
At any rate, four days is not a long time. These things take time to heal. Put ice on it if it feels hot and sore. Put heat on it if it is stiff and sore. Gently stretch it but do not strain it with too many stairs or jumping etc. Be patient. 'Tincture of time' is great medicine.
Good Luck.


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