I hear that tires rot out, from the inside, after 4 years of use or not use.
And where did you hear that nonsense . Go to the Tire Rack web site and all of your questions will be answered.
No, just No. 10 years maybe, not 4. Unless, of course, the tires are $29 super cheapy tires. Those were rotten when you put them on! 
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A myth that is disproven at every tire shop every single day.
Take a look at:
Technically, tires don’t rot, which implies a biological agent. They do have various chemicals out-gas and they are deteriorated by ultraviolet light and ozone. But these factors occur on the outside of the tires long before there is deterioration on the inside. Obviously, there is no ultraviolet exposure at all on the inside of the tire.
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And let me add that for practical purposes, new tires, properly stored, age very slowly. 3 years has been documented as the same as 0 days, but it is common in the tire industry to cite 6 years.
However, once in service, my quote above applies.
My tires are 16 years’ old.
We don’t know where you “heard” this, but tires last a lot longer than that. Just the same, the ozone in the air will gradually destroy the outside of a a tire. I had an unused spare on my camper crack in 12 years from sitting in the sun.
The rot part you heard is just “rot” as the British say.
Some years ago oil explorers in North Africa found an old WW II army truck that had been buried in the sand for at least 60 years. And yes, the tires were intact and still inflated! No rot there.
Most automotive “truths” are spouted near closing time in bars and road houses.
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Yes and we’ve beaten that horse not just to death but to a bloody mush.
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Have to admit I got a roofing nail in my sidewall the night before leaving on a 500 mile road trip. No shop was able to fix it in time as I noticed it at 4:30 friday afternoon and we planned on taking off 7 am Sat morning. So I put on the 15 year old spare and made the trip at interstate speeds I knew it was not the best scenario, but did it anyway no problem. Got a replacement tire for the one with the nail in the sidewall, they only had 4k miles. I tell @RandomTroll get some new tires, but I would be the pot calling the kettle whatever the PC correct term is today.
… and I have an old friend from my undergraduate days who brags that he only bathes once a week. 
(Hint: Neither his statement, nor yours, represents behavior that most people should emulate.)
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Ozone content varies: there’s a lot in Los Angeles, not so much in Fargo.
You win! I wasn’t bragging but offering my experience.
Whether he needs it or not. I shower at least once daily. When I was a kid our parents bathed us Saturday night. A friend reported that in her family they used the same wash water for their weekly baths.