Unfortunately Microsoft needs 3rd parties to survive in this industry and they won´t let them go away with a 4 year life cicle.
In this business most machines have their greates time during their third year - but in a 4 year life cicle the 3rd year will be the year the next machine is hyped --> no big games, no profits, no big 3rd year, no time to get used to the system, during 4 years it is impossible to seel enough units to make the business worth it. After 4 years the PS2 had 75 million units sold - just imagine what would happen with higher developement costs but less consoles sold?
No, it wouldn´t happen and hurt the industry too much.