Bonds are so boring. People that invest in 0.5% interest CDs or bonds with any large portion of their income are WAY too risk adverse. You may indeed not lose net money, but you are guaranteed to have less spending power over time AND guaranteed not to gain money....why live like that...?
The simplest "safe enough" cash earners are dividend focused ETFs (average ~3-4% dividend yield, investing in large staple companies that tend to not have extreme price swings). In that scenario your $1,000,000 investment could crank out $30,000 / year in dividends, and on top of that the lump sump will also grow over time. Over the past 10 years Vanguards VYM tripled in value while paying out a 3% dividend, so if you retired in 2010 with $1,000,000 in there you would have started with a $30,000 dividend, and today you'd have $3,000,000 invested in there and be getting a $90,000 dividend - so after 10 years of retirement your Stache tripled and you bumped up your lifestyle into Spendypants territory guilt free! And yes, the past decade has been extreme for capital growth, but the past 100 years that dividend payout would still have worked.
Another income generator would be a series of CEFs (something I don't really see often on the MMM pages). These pay out a "dividend" in the range of 6-8%, but the asset price doesn't change as much over time so it won't grow with inflation really (unless of course you only keep 3-4% of the payout, and re-invest the rest!). In this scenario your $1,000,000 would generate ~$70,000 / year in cash flow.
And then there is another root MMM quality - ADAPT! If you FIRE on a tight budget and the market crashes, tighten your belt further and / or find a temporary job - not the end of the world.
Personally I'm somewhere between a SWAMI and a "one more year" and a "too chicken to ever likely pull the trigger" and "don't hate my job and would get bored with 40 more hours of time", so I'll likely FIRE needing like a 2% SWR, or maybe even less - who knows. My plan is something like 50% in dividend ETFs and CEFs to hit my cash flow, 25% in broad market ETFs to be boring, 15-20% in individual stocks because YOLO, and 5-10% in cash that I may or may not put in CDs because is 0.5% interest really worth the hassle / not being able to take it out if you need it?
Anyhow. I think the 4% rule is fine, as long as you accept the fact you need to invest in stocks and not bonds, and there is a non-zero chance that you'll need to go back to work at least temporarily.