It's pretty much impossible to say how long the system will last, the only thing you can control is the specs of the hardware you pick.
Hardware breaks, especially since you want a tablet/laptop which adds a while bunch of other things to break like the keyboard to tablet connection and specialized hardware to make the thing sleek.
Basically if you are looking for a laptop for 4+ years you need to go to the higher end of specs today.
Take a look at the Lenovo Yoga
http
/tablet-pc-review.toptenreviews.com/lenovo-yoga-review.html