If you have begun jogging after a long break, then you should prime your body. Don't start directly with 4 km. Start with 1.5 km for a week, and increase your distance slowly over a period of 2-3 weeks. What has happened is that because your muscles are in continuous need of oxygen
during such sudden, strenuous exercise, and your body is not able to supply it, it switches to anaerobic mode of energy production (wherein oxygen is not needed). Kind of like taking a detour from the usual path.
The problem here is that because of this alternate mode, a lot of lactic acid gets produced in the body as a by-product and gets stored in your muscles. This gives rise to pain, because lactic acid gets separated from your muscles very slowly. Your back muscles have gone into a spasm.
Do some hot fomentation, take rest from jogging for 2-3 days or till the pain subsides. Then begin slowly. Don't worry, you'll get back to your 4 km run slowly, albeit, a bit slowly.
All the best!