<<And before someone says valuation isn’t a concern for his wife, in an interview he did say “anyone will do fine with that”.>>

I would take that comment as an approximation : )
Certainly anyone who doesn't really have enough money saved to be able to afford that strategy is not going to do fine, so it certainly doesn't work for "anyone".

My comment was aimed at those who will inevitably say Buffett's wife will have so much money it doesn't matter what she invests it in.

Here's the full Buffett quote:
3/3/2014
BUFFETT: Well, I didn't lay out my whole will. There's hope for some of you who haven't been mentioned yet.
The-- but I did explain, because I laid out what I thought the average person who is not an expert on stocks should do.
And my widow will not be an expert on stocks. And- I wanna be sure she gets a decent result. She isn't gonna get a sensational result, you know? And since all my Berkshire shares are going-- to philanthropy-- the question becomes what does she do with the cash that's left to her? And I've been-- part of it goes outright, part of it goes to a trustee. But I've told the trustee to put 90% of it in an S&P 500 index fund and 10% in short-term governments. And the reason for the 10% in short-term governments is that if there's a terrible period in the market and she's withdrawing 3% or 4% a year you take it out of that instead of selling stocks at the wrong time. She'll do fine with that. And anybody will do fine with that. It's low-cost, it's in a bunch of wonderful businesses and it takes care of itself.

Here's a link to the video if interested, the snippet transcribed above starts around the 8:59 mark.
Buffett: "People react too much to short-term things" - full interview (cnbc.com)
buffett.cnbc.com - Buffett people react too much to short term things full interview

As for the 4% rule being applied to a SPY type portfolio, I wouldn't recommend doing it naively. The number of years it will last is a very strong function of valuation levels on the day of retirement. Some folks who do that will go broke because their initial estimate of the sustainable withdrawal rate will be too high, because valuations were high when they started the program. I would propose the single adjustment that the SWR be set to equal the cyclically adjusted earnings yield of the index, for example using ten years of inflation adjusted earnings. It would have been 4% at the start of 1996, and again in August 2002, but would have been only 2.1% in between at the market top in March 2000. About 2.6% at the moment.

I agree, Buffett's strategy is not going to always work, valuation is a factor, and sequence of returns. And how do you decide when to pull from cash and when to sell some stock? But even starting in 2000 it hasn't done that badly. Our 2000 retiree naively pulling out his inflation adjusted 4% from 90% SPY/10% cash still has 4 years of withdrawals left.

testfol.io: Portfolio Backtester for ETFs and Asset Allocation

Advice to a trustee is very tricky. Would Buffett change it in light of SPY over-valuation? No, I don't think so. You can't change it once you're gone, after all.

Been struggling with this myself - what advice to leave for my trustee, who will not be an expert on investing? I'll be turning in my urn if they hand it over to a "helper" charging 1 or 2%.