Well there are many things that keep you on the road. What about track rod ends, suspension joints, shock absorbers, steering rack, pinion, shaft and steering wheel?
These one-liners are straight out of the ads cooked up by the branded tyre companies.
Throwing away a tyre with 4mm left is just a waste of money and environmentally unfriendly. It does depend really on whether its a pristine car and if you want to drive it to the limit, but if its an oldish car and you just pootle about in it, just replace the worn one.