Mac Pro and MacBook Pro are not in the same league, as one is upgradeable with HDDs, RAM, GPUs, expansion cards, maybe even CPU, ..., the other only with RAM and HDD.
One is portable, one can be carried with a small bag. One is heavy, the other one is heavier. One is supporting more than two displays, one has one built in.
Both Macs cater to different purposes and needs. And both will be still useable in four years, just perceivably slower due to experience and knowledge of newer technology.