Pain in calf after sclerotherapy. Can anyone advise me what it could be and how long it could take to clear?

This can all be normal but you definitely need a ultrasound to rule out a deep vein thrombosis.  A 4 cm vein required a large volume of sclerosing agent and cause pain as the vein fibrosis and scars down.  I recommend Ice, compression, elevation and NSAID's for pain.  It should improve over the next coulple of weeks.  

What you need is a venous ultrasound to ensure that there is no DVT which could happen following sclerotherapy of varicose veins.  Most likely you have pain from the treated varicose vein which has now clotted and will improve over time with heat to the area and ibuprofen.  However, a DVT needs to be ruled out.  Follow up with your treating physician.