The 4Mbps is pretty much break even for them.
Each Mbps higher you go is cheaper than the last one for them, so they try and start their packages at 20Mbps.
https://shop.vumatel.co.za/packages/all
is what you're looking for in regards to checking all Vumatel packages.
Most 20Mbps uncapped are cheaper than OpenServe 10mbps uncapped.