Is it common to have pain in calf/leg muscles after being in a walking boot for 4 weeks and just starting to walk again without it?

Yes: Yes, this is common to have since you haven't moved this particular muscle in a month. That being said, not moving a limb can increase your risk for blood clot. If you have swelling, redness of the skin, feeling of a cramp in the muscle, get to a doctor or ER right away to make sure you do not have a clot. These are all signs for a blood clot.