As a fully qualified tradesman, I know a bucket load of plumbers/gas men and 1000-1500 (600-700) mates rates. Depends on what model you want, instantaneous/intelligent/remote controls and if the pipework for said option is available. Often a plumber will have to do some work on rerouting piping before the boiler can be installed.
4 mil is waaaay too much.
Coming from an electrical perspective, If I have to change a consumer unit and I need to add cabling. I need to do that work before I can connect them to the power. That't the only reason I can see the price being that high.