Had to pay R250 for 4kg extra

I checked in this aftertoon with safair. I had originally booked online for single check in bag, for 20KG. At the airport my bag weighed 24KG. I dont mind paying extra since it was over but yet i had to book another bag in ,which i did not have, and pay 250 for an extra 20kg whereas I only had an extra 4 . Is that fair ?? This is a flawed system. I am a student in Cape Town and live in JHB . I fly back and forth numerous times in the year and now I'm serious looking at using other airlines during the rest of my 5 year degree. On the plus side the staff were friendly.