I had a play around with the different options on "printerland" (a seller of printers who quote "running costs"* for all the printers they sell) and I think your work got a very good deal, but there also seems to be a big running cost difference between big office multi-function devices and more affordable desktop printers and there also seems to be big variation between printers in the same price range. 4p per page seems absoloutely plausible for a cheap printer from a manufactuer known for high running costs.For comparison, at work our contract is for 2.36p for colour and 0.27p for mono. Admittedly that is a heavy duty printer but I am surprised yours is ten times the cost!
* Based on the prices of toner cartridges, does not include the cost of other longer-life consumables.