The bull market nobody saw coming turns four years old this week, powered by an artificial-intelligence boom that has created trillions of dollars in new investor value, launched a game-changing drive to develop new technology, and raised big questions about the risks of a bubble that could tear the rally down at a moment's notice.

The S&P 500 bottomed out from a vicious bear market in October 2022, when it tumbled nearly 25% from its prior peak following Russia's invasion of Ukraine, which prompted a global surge in oil prices. Investors were also contending with a Federal Reserve intent on normalizing interest rates after the central bank made emergency cuts tied to the Covid pandemic.

Its Oct. 12, 2022, close of 3,577.03 points, however, marked the start of a relentless bull run that has produced 123 new closing highs, added nearly $40 trillion in overall market value, and helped spur four tech giants -- Nvidia, Microsoft, Apple, and Google parent Alphabet -- to each have market capitalizations of at least $4 trillion.

And it may have only reached the halfway point.

Carson Group's chief market strategist, Ryan Dietrick, writes that of the five major bull markets of the past half-century that have powered into a fourth year, all have made it to a fifth. In fact, the average length of a bull market is normally eight years.

"We aren't saying this bull is only halfway over, but we are saying to be open to this one lasting a lot longer," he said in a note to clients.

The S&P 500 has risen nearly 120% over the past four years -- with the Nasdaq Composite surging more than 165% in the same period -- as part of a tech-led bull market that few saw coming when it began to gather steam in the autumn of 2022.

The market got a jolt in November of that year, when OpenAI launched its ChatGPT chatbot -- a debut that helped spur a multi-trillion-dollar AI investment race that has transformed markets and realigned the global economy.

Microsoft's investment in OpenAI -- which is now valued at more that $1.4 trillion, based on OpenAI's reported plans to become a publically-traded company next year -- sparked the early stage of the bull-market rally, but Nvidia's first-quarter earnings report in spring 2024 added the real rocket fuel.

The chip maker said that during the period, data-center revenue topped its gaming sales for the first time ever and its top line rose 83% year over year to $3.75 billion, and it forecast a next-quarter tally of more than $11 billion.

Nvidia, which had a market cap of about $750 billion the day before the earnings release, was suddenly worth more than $1 trillion by the end of the same month. As of October 2026, Nvidia's market cap is just shy of $6 trillion.

Nvidia's outlook was in many respects the first real evidence of the AI investment boom, which is expected to top $1 trillion in new cash this year, and as much as $7 trillion globally by the end of the decade, according to Goldman Sachs estimates.

And virtually everyone in the AI tech space has joined the fun. A CNBC index of the Magnificent Seven stocks, which includes Nvidia, has risen more than 150% since the bull market began, while stocks in the Philadelphia Semiconductor index have nearly quintupled and hit a record high in late June.

In fact, the top 10 S&P 500 stocks, all of them tech-focused, now comprise around 40% of the benchmark's $70 trillion in market value. In 2022, the entire tech sector made up less than one-third of the S&P's $30 trillion in value.

The number of companies joining the rally has also been impressive: 67% of stocks in the S&P 500 have traded above their 200-day moving average, a key Wall Street benchmark, during the entirety of the current bull market, according to data from LPL Financial.

That participation has narrowed of late, with tech pacing nearly all of the benchmark's gains since the turn of the summer, although some analysts see more stocks participating in the rally as the S&P 500 hits new highs.

"The next key test for the bull market is whether participation continues to broaden beyond big tech and AI," said LPL's chief cross-asset strategist, Adam Turnquist. "A sustained expansion in market breadth measures would help validate the breakout and improve the odds the rally to new highs is durable."

Breadth, however, might be the least of the market's worries heading into the bull market's fifth year, as global crude prices remain high, bond markets remain volatile, and the Federal Reserve turns hawkish amid creeping inflation risks.

Still, if the S&P 500 -- which is up roughly 14% this year -- climbs higher into November and ekes out gains into December, the current bull market will go from ninth place to seventh on the list of 27 20%-plus runs for the benchmark since 1928.

That said, the index has some ways to go before nearing the gains of the record rally that ended in 2000 after nearly 4,500 days, or the post-financial crisis rally that lasted nearly 4,000 days from March 2009 to the Covid pandemic pullback in the winter 2020.

But some of the early signals are positive.

The S&P 500 is a lot cheaper than it was in January, when investors were paying more than 22 times for every dollar of forward earnings, compared with around 19.5 times ahead of Monday's bull market anniversary, and profit growth remains impressive.

"I don't mind multiple compression because it tells me that this bull market is sustainable and [that compression] elongates it," said Nancy Tengler, chief investment officer at Laffer Tengler Investments.

"I like where we sit for this bull market, and earnings growth does ultimately drive stock price performance," she added. "I think we have some room to run, certainly for the fourth quarter and into next year."

Graduating from the bull market that nobody expected to the one that everyone continues to bet on might seem worrying -- but fighting the tape hasn't paid the bills in quite some time.