Re: The correct form of present perfect...
They're all a bit clunky, to be honest. The most likely versions (if the model has not been completed) are:
Originally Posted by Mehrgan
It's been four years since I started working on this model.
I've been working on this model for four years.
If the model has been completed, then:
I worked on that model for four years [before I finally completed it].
I was working on that model for four years [before I finally completed it].
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.