They are two different types of funds. You're comparing apples to oranges.
The total stock market index (FSTVX) is 100% equities and it's all U.S.-based funds to boot. Fidelity 4in1 (FFNOX) is a blended fund made up of a bit of total U.S. stock market, some international stock market, a bit of bonds and a tiny dash of cash...
Of course FSTVX going to look better when the U.S. market is doing well overall because pure stocks always outperform blended funds. Bond/cash are a drag when the market is good. You hold them because when the market crashes or corrections happen, they switch gears and start growing and halt the plunge of your total portfolio - allowing you to have a safety net as it were to draw from in times of poor stock performance.
If your allocation says "I want to have the safety of bonds because REASONS" then you don't hold 100% stock/equities. You can use FSTVX for your stocks, and go select a nice solid bond index fund to fill up your asset allocation and self-manage your own portfolio, but FFNOX is a cheap blended index fund with a gross ER of 0.13%, decent performer when compared to other blended index funds and the simplicity of holding one fund that has a blend of what you want overall is quite convenient. It has a low turnover, but there are some pretty high $$ of dividends/cap gains generated, so it isn't tax efficient as some other vehicles and I'd likely want to make sure to hold it in a tax deferred account if possible.
I like FFNOX due to its cheapness and simplicity, but I personally hold individual funds like FSTVX and Fidelity U.S. Bond Index Fund (FSITX) for bonds in my case because they are even more cheap to hold and I prefer to self manage anyway and can tweak my holdings easily.