The 4-Year Stock Bull Market Runs on Big Tech — Yields Could Decide What's Next
The S&P 500 bull market turns four years old on Monday, Oct. 12. The Price Index is up 117% from its bear-market closing low Morningstar. It was trading around record highs ahead of the anniversary Reuters.
The count starts at 3,577.03, the closing low where the advance began Yahoo Finance. MarketWatch's Oct. 11 note, "The bull market is turning 4 years old — but yields could crash the party", points to the same 3,577 level. S&P Dow Jones Indices defines a bull market as a 20% rise in the S&P 500 Price Index from its prior low. The run ends when the index sets a high and then falls 20% S&P Global.
For context on why that 20% rule matters, technicians use it as the scorekeeper. Under that rule, this advance stays intact until a 20% decline from a confirmed peak actually prints.
Leadership stayed narrow. The primary trend remains positive, led by the tech sector and AI-related megacaps, the largest companies tied to artificial intelligence Morningstar. Reuters, in "AI-centered US stock bull market nears four-year anniversary", describes the same setup Reuters. Gains in those large tech and AI stocks have given them more sway over major stock indexes but also made the market top-heavy.
Concentration defines the advance
The combined weight of the five largest S&P 500 companies hit multi-decade highs in July 2025 S&P Global. In a cap-weighted index, where bigger companies take a bigger share of the index, that skew works in both directions. It lifts index returns when megacaps outperform. It deepens index losses when they lag.
The broader context here is breadth, meaning how many stocks take part in the rally. A top-heavy index can set new highs on a handful of names while the typical stock stalls. That lifts the Price Index but narrows the base holding it up. For portfolio construction, it raises tracking error, the gap between a portfolio and its benchmark, around any shift toward equal-weight, mid-cap or value holdings.
A rally in 2026 would make it four straight calendar years of gains for U.S. stocks, the longest winning streak in nearly two decades Bloomberg.
In my view, streaks like that change the math going forward. Long winning runs compress assumptions about future returns. They do not set the timing for what comes next.
Expectations ahead of Oct. 12 were split. Forecasts cited in early October 2026 called for the S&P 500 to end 2026 at or above 8,000, with the bull extending into next year Reuters. Panmure Liberum forecast differently. It forecast a decline in the S&P 500 and warned the bull run was nearing an end.
What matters for savers and investors is how far apart those views sit. The gap is unusually wide for year four of an advance. That leaves positioning sensitive to small surprises on earnings, policy and rates.
Yields are the swing variable
MarketWatch frames rates as the possible break. Its Oct. 11 story points to yields rising to 6%. A related Oct. 9 piece, "Here's how Treasury yields could rise to 6%", lays out a path that does not require market upheaval MarketWatch. The framing is conditional. It is a scenario for long-term borrowing costs, not a level already reached. A Treasury yield is the annual return an investor earns for holding government debt.
The reference points for that debate are daily and standard. The Federal Reserve's H.15 Selected Interest Rates (Daily) release dated Oct. 9, 2026 includes a yield for 10-year maturity Federal Reserve. The U.S. Treasury publishes Daily Treasury PAR Yield Curve Rates that link par yield, the rate that prices a bond at face value, to time to maturity using closing market data U.S. Treasury. Treasury says it estimates a par yield for a 10-year maturity each day.
The starting point for any move toward 6% stood at these levels by summer. A U.S. Treasury report said 10-year yields had risen to roughly 4.6% and two-year yields to around 4.2%, above a 3.50%-3.75% target range U.S. Treasury.
To put that rate backdrop in context, long-term borrowing costs sold off even as short-term expectations stayed tied to cuts.
The broader context here is how sensitive high-growth stocks are to discount rates, the rates used to value future profits today. A joint rise in real yields, returns after inflation, and term premium, the extra return for holding longer debt, hits megacap values first. It then feeds through index math because those names dominate weight. Credit spreads, the extra yield on corporate bonds over Treasuries, can stay calm while stocks reprice. That split often muddles cross-asset signals late in a bull.
Looking at what this means for risk management, the question is not whether 6% prints. It is how much valuation compression, shrinking price-to-earnings multiples, the market absorbs before earnings growth offsets it. A slow drift higher lets earnings estimates, buybacks and rotation cushion the Price Index. A fast repricing tightens financial conditions quickly. With concentration at multi-decade highs and the index near records, the cushion from breadth is thin. That structure is why a four-year-old bull led by AI megacaps still hinges on the rates tape.
