Should i be worried i've had a pain in my left side of the back of my rib cage at the bottom. i've had this pain for4 days.

Pain for 4 days: It depends if the pain is getting better or not. If it is getting better, then you should not be worried. If it is not getting better, then it becomes a concern. See if it is better or gone in the next couple of days. If it is not better or worsens, then I would recommend that you get it evaluated.

Side pain: Usual cause for this would likely be muscular-skeletal pain. More concerning would be a possible kidney stone. Muscle pain typically worsens with movement even light movement occasionally while kidney stone pain is more vague, constant unrelenting discomfort not usually worsened by movement so classically leading patients to move a lot for relief. Infection is also possible. Urine test would help.